aging

The effects of ageing on a human face

An elderly woman

Ageing (British English) or aging (American and Canadian English) is the accumulation of changes in an organism or object over time. Ageing in humans refers to a multidimensional process of physical, psychological, and social change. Some dimensions of ageing grow and expand over time, while others decline. Reaction time, for example, may slow with age, while knowledge of world events and wisdom may expand. Research shows that even late in life potential exists for physical, mental, and social growth and development. Ageing is an important part of all human societies reflecting the biological changes that occur, but also reflecting cultural and societal conventions. Age is usually measured in full years — and months for young children. A person's birthday is often an important event. Roughly 100,000 people worldwide die each day of age-related causes.^[1]

The term "ageing" is somewhat ambiguous. Distinctions may be made between "universal ageing" (age changes that all people share) and "probabilistic ageing" (age changes that may happen to some, but not all people as they grow older, such as the onset of Type Two diabetes). Chronological ageing, referring to how old a person is, is arguably the most straightforward definition of ageing and may be distinguished from "social ageing" (society's expectations of how people should act as they grow older) and "biological ageing" (an organism's physical state as it ages). There is also a distinction between "proximal ageing" (age-based effects that come about because of factors in the recent past) and "distal ageing" (age-based differences that can be traced back to a cause early in person's life, such as childhood poliomyelitis).^[2]

Differences are sometimes made between populations of elderly people. Divisions are sometimes made between the young old (65-74), the middle old (75-84) and the oldest old (those aged 85 and above). However, problematic in this is that chronological age does not correlate perfectly with functional age, i.e. two people may be of the same age, but differ in their mental and physical capacities. Each nation, government and non-government organization has different ways of classifying age.

Population ageing is the increase in the number and proportion of older people in society. Population ageing has three possible causes: migration, longer life expectancy (decreased death rate), and decreased birth rate. Ageing has a significant impact on society. Young people tend to commit most crimes, they are more likely to push for political and social change, to develop and adopt new technologies, and to need education. Older people have different requirements from society and government as opposed to young people, and frequently differing values as well. Older people are also far more likely to vote, and in many countries the young are forbidden from voting. Thus, the aged have comparatively more political influence.

Senescence

A map showing median age figures for 2001

An elderly man

In biology, senescence is the state or process of ageing. Cellular senescence is a phenomenon where isolated cells demonstrate a limited ability to divide in culture (the Hayflick Limit, discovered by Leonard Hayflick in 1965), while Organismal senescence is the ageing of organisms. After a period of near perfect renewal (in humans, between 20 and 35 years of age), organismal senescence is characterized by the declining ability to respond to stress, increasing homeostatic imbalance and increased risk of disease. This irreversible series of changes inevitably ends in death. Some researchers (specifically biogerontologists) are treating ageing as a disease. As genes that have an effect on ageing are discovered, ageing is increasingly being regarded in a similar fashion to other genetic conditions, potentially "treatable."

Indeed, ageing is not an unavoidable property of life. Instead, it is the result of a genetic program. Numerous species show very low signs of ageing ("negligible senescence'), the best known being trees like the bristlecone pine (however Dr. Hayflick states that the bristlecone pine has no cells older than 30 years), fish like the sturgeon and the rockfish, invertebrates like the quahog or sea anemone ^[3].

In humans and other animals, cellular senescence has been attributed to the shortening of telomeres with each cell cycle; when telomeres become too short, the cells die. The length of telomeres is therefore the "molecular clock," predicted by Hayflick. Telomere length is maintained in immortal cells (e.g. germ cells and keratinocyte stem cells, but not other skin cell types) by the enzyme telomerase. In the laboratory, mortal cell lines can be immortalized by the activation of their telomerase gene, present in all cells but active in few cell types. Cancerous cells must become immortal to multiply without limit. This important step towards carcinogenesis implies, in 85% of cancers, the reactivation of their telomerase gene by mutation. Since this mutation is rare, the telomere "clock" can be seen as a protective mechanism against cancer ^[4]. Research has shown that the clock must be located in the nucleus of each cell and there have been reports that the longevity clock might be located in genes on either the first or fourth chromosome of the twenty-three pairs of human chromosomes.

Other genes are known to affect the ageing process, the sirtuin family of genes have been shown to have a significant effect on the lifespan of yeast and nematodes. Over-expression of the RAS2 gene increases lifespan in yeast substantially.

In addition to genetic ties to lifespan, diet has been shown to substantially affect lifespan in many animals. Specifically, caloric restriction (that is, restricting calories to 30-50% less than an ad libitum animal would consume, while still maintaining proper nutrient intake), has been shown to increase lifespan in mice up to 50%. Caloric restriction works on many other species beyond mice (including species as diverse as yeast and Drosophila), and appears (though the data is not conclusive) to increase lifespan in primates according to a study done on Rhesus monkeys at the National Institute of Health (US), although the increase in lifespan is only notable if the caloric restriction is started early in life. Since, at the molecular level, age is counted not as time but as the number of cell doublings, this effect of calorie reduction could be mediated by the slowing of cellular growth and, therefore, the lengthening of the time between cell divisions.

Drug companies are currently searching for ways to mimic the lifespan-extending effects of caloric restriction without having to severely reduce food consumption.

In his book, 'How and Why We Age', Dr. Hayflick notes a contradiction to the caloric restriction longevity increase theory for humans, noting that data from the Baltimore Longitudinal Study of Aging show that being thin does not favour longevity.

Dividing the lifespan

An animal's life is often divided into various ages. Historically, the lifespan of humans is divided into Eight ages; because biological changes are slow moving and vary from person to person, arbitrary dates are usually set to mark periods of life. In some cultures the divisions given below are quite varied.

In the USA, adulthood legally begins at the age of eighteen, while old age is considered to begin at the age of legal retirement (approximately 65).

• Pre-conception: ovum, spermatozoon
• Conception: fertilization
• Pre-birth: conception to birth (pregnancy)
• Infancy: Birth to 1
• Childhood: 1 to 12
• Adolescence: 13 to 19
• Early adulthood: 20 to 39
• Middle adulthood: 40 to 64
• Late adulthood: 65+
• Death
• Post-Death: Decomposition of the body

Ages can also be divided by decade:

People from 13 to 19 years of age are also known as teens or teenagers. The casual terms "twentysomething", "thirtysomething," etc., are also in use to describe people by decade or age.

Cultural variations

In some cultures (for example Serbian) there are four ways to express age: by counting years with or without including current year. For example, it could be said about the same person that he is twenty years old or that he is in the twenty-first year of his life. In Russian the former expression is generally used, the latter one has restricted usage: it is used for age of a deceased person in obituaries and for the age of an adult when it is desired to show him/her younger than he/she is. (Psychologically, a woman in her 20th year seems older than one who is 19 years old.)

Considerable letters of cultures have less of a problem with age compared with what has been described above, and it is seen as an important status to reach stages in life, rather than defined numerical ages. Advanced age is given more respect and status.

East Asian age reckoning is different from that found in Western culture. Traditional Chinese culture uses a different ageing method, called Xusui (虛歲) with respect to common ageing which is called Zhousui (周歲). In the Xusui method, people are born at age 1, not age 0, because conception is already considered to be birth.

Society

Legal

There are variations in many countries as to what age a person legally becomes an adult.

Most legal systems define a specific age for when an individual is allowed or obliged to do something. These ages include voting age, drinking age, age of consent, age of majority, age of criminal responsibility, marriageable age, age of candidacy, and mandatory retirement age. Admission to a movie for instance, may depend on age according to a motion picture rating system. A bus fare might be discounted for the young or old.

Similarly in many countries in jurisprudence, the defence of infancy is a form of defence by which a defendant argues that, at the time a law was broken, they were not liable for their actions, and thus should not be held liable for a crime. Many courts recognize that defendants who are considered to be juveniles may avoid criminal prosecution on account of their age, and in borderline cases the age of the offender is often held to be a mitigating circumstance.

Economics and marketing

The economics of ageing are also of great importance. Children and teenagers have little money of their own, but most of it is available for buying consumer goods. They also have considerable impact on how their parents spend money.

Young adults are an even more valuable cohort. They often have jobs with few responsibilities such as a mortgage or children. They do not yet have set buying habits and are more open to new products.

The young are thus the central target of marketers.^[5] Television is programmed to attract the range of 15 to 35 year olds. Mainstream movies are also built around appealing to the young.

Health care demand

Many societies in the rich world, e.g. Western Europe and Japan, have ageing populations. While the effects on society are complex, there is a concern about the impact on health care demand. The large number of suggestions in the literature for specific interventions to cope with the expected increase in demand for long-term care in ageing societies can be organized under four headings: improve system performance; redesign service delivery; support informal caregivers; and shift demographic parameters.^[6]

However, the annual growth in national health spending is not mainly due to increasing demand from ageing populations, but rather has been driven by rising incomes, costly new medical technology, a shortage of health care workers and informational asymmetries between providers and patients.^[7]

Even so, it has been estimated that population ageing only explains 0.2 percentage points of the annual growth rate in medical spending of 4.3 percent since 1970. In addition, certain reforms to Medicare decreased elderly spending on home health care by 12.5 percent per year between 1996 and 2000. ^[8] This would suggest that the impact of ageing populations on health care costs is not inevitable.

Impact on Prisons

As of July 2007, medical costs for a typical inmate in the USA might run an agency around US$33 per day, while costs for an ageing inmate could run upwards of US$100. Most State DOCs report spending more than 10 percent of the annual budget on elderly care. That is expected to rise over the next 10–20 years. Some states have talked about releasing ageing inmates early. ^[9]

Cognitive effects

Steady decline in many cognitive processes are seen across the lifespan, starting in one's thirties. Research has focused in particular on memory and ageing, and has found decline in many types of memory with ageing, but not in semantic memory or general knowledge such as vocabulary definitions, which typically increases or remains steady. Early studies on changes in cognition with age generally found declines in intelligence in the elderly, but studies were cross-sectional rather than longitudinal and thus results may be an artefact of cohort rather than a true example of decline. Intelligence may decline with age, though the rate may vary depending on the type, and may in fact remain steady throughout most of the lifespan, dropping suddenly only as people near the end of their lives. Individual variations in rate of cognitive decline may therefore be explained in terms of people having different lengths of life.^[2] There are changes to the brain: though neuron loss is minor after 20 years of age there is a 10% reduction each decade in the total length of the brain's myelinated axons.^[10]

Coping and well-being

Psychologists have examined coping skills in the elderly. Various factors, such as social support, religion and spirituality, active engagement with life and having an internal locus of control have been proposed as being beneficial in helping people to cope with stressful life events in later life.^[11][12][13] Social support and personal control are possibly the two most important factors that predict well-being, morbidity and mortality in adults.^[14] Other factors that may link to well-being and quality of life in the elderly include social relationships (possibly relationships with pets as well as humans), and health.^[15]

Individuals in different wings in the same retirement home have demonstrated a lower risk of mortality and higher alertness and self-rated health in the wing where residents had greater control over their environment,^[16][17] though personal control may have less impact on specific measures of health.^[13] Social control, perceptions of how much influence one has over one's social relationships, shows support as a moderator variable for the relationship between social support and perceived health in the elderly, and may positively influence coping in the elderly.^[18]

Religion

Religion has been an important factor used by the elderly in coping with the demands of later life, and appears more often than other forms of coping later in life.^[19] Religious commitment may also be associated with reduced mortality,^{[citation needed]} though religiosity is a multidimensional variable; while participation in religious activities in the sense of participation in formal and organized rituals may decline, it may become a more informal, but still important aspect of life such as through personal or private prayer.^[20]

Self-rated health

Self-ratings of health, the beliefs in one's own health as excellent, fair or poor, has been correlated with well-being and mortality in the elderly; positive ratings are linked to high well-being and reduced mortality.^[21][22] Various reasons have been proposed for this association; people who are objectively healthy may naturally rate their health better than that of their ill counterparts, though this link has been observed even in studies which have controlled for socioeconomic status, psychological functioning and health status.^[23] This finding is generally stronger for men than women,^[22] though the pattern between genders is not universal across all studies, and some results suggest sex-based differences only appear in certain age groups, for certain causes of mortality and within a specific sub-set of self-ratings of health.^[23]

Retirement

Retirement, a common transition faced by the elderly, may have both positive and negative consequences.^[24]

Societal impact

Of the roughly 150,000 people who die each day across the globe, about two thirds — 100,000 per day — die of age-related causes.^[1] In industrialized nations, the proportion is much higher, reaching 90%.^[1]

Societal ageing refers to the demographic ageing of populations and societies.^[25] Cultural differences in attitudes to ageing have been studied.^{[citation needed]}

Emotional improvement

Given the physical and cognitive declines seen in ageing, a surprising finding is that emotional experience improves with age.^{[citation needed]} Older adults are better at regulating their emotions and experience negative affect less frequently than younger adults and show a positivity effect in their attention and memory.^{[citation needed]} The emotional improvements show up in longitudinal studies^[specify] as well as in cross-sectional studies^[specify] and so cannot be entirely due to only the happier individuals surviving.

Terminology

The concept of successful ageing can be traced back to the 1950s, and popularised in the 1980s. Previous research into ageing exaggerated the extent to which health disabilities, such as diabetes or osteoporosis, could be attributed exclusively to age, and research in gerontology exaggerated the homogeneity of samples of elderly people.^[26][27]

Successful ageing consists of three components:^[28]

1. Low probability of disease or disability;
2. High cognitive and physical function capacity;
3. Active engagement with life.

A greater number of people self-report successful ageing than those that strictly meet these criteria.^[26]

Successful ageing may be viewed an interdisciplinary concept, spanning both psychology and sociology, where it is seen as the transaction between society and individuals across the life span with specific focus on the later years of life.^[29] The terms "healthy aging"^[26] "optimal aging" have been proposed as alternatives to successful ageing.

Six suggested dimensions of successful ageing include:^[13]

1. No physical disability over the age of 75 as rated by a physician;
2. Good subjective health assessment (i.e. good self-ratings of one's health);
3. Length of undisabled life;
4. Good mental health;
5. Objective social support;
6. Self-rated life satisfaction in eight domains, namely marriage, income-related work, children, friendship and social contacts, hobbies, community service activities, religion and recreation/sports

Theories

Biological theories

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At present, the biological basis of ageing is unknown. Most scientists agree that substantial variability exists in the rates of ageing across different species, and that this to a large extent is genetically based. In model organisms and laboratory settings, researchers have been able to demonstrate that selected alterations in specific genes can extend lifespan (quite substantially in nematodes, less so in fruit flies, and even less in mice). Nevertheless, even in the relatively simple organisms, the mechanism of ageing remain to be elucidated. Because the lifespan of even the simple lab mouse is around 3 years, very few experiments directly test specific ageing theories (most of the evidence for the ones listed below is correlative).

The National Institute on Aging currently funds an intervention testing program, whereby investigators nominate compounds (based on specific molecular ageing theories) to have evaluated with respect to their effects on lifespan and age-related biomarkers in outbred mice ^[30]. Previous age-related testing in mammals has proved largely irreproducible, because of small numbers of animals, and lax mouse husbandry conditions. The intervention testing program aims to address this by conducting parallel experiments at three internationally recognized mouse ageing-centres, the Barshop Institute at UTHSCSA, the University of Michigan at Ann Arbor and the Jackson Lab. While the hypotheses below reflect some of the current direction in biological ageing research, none of them is accepted as a "theory" in the sense of the "theory of gravity" or "theory of relativity".

Telomere Theory

Telomeres (structures at the ends of chromosomes) have experimentally been shown to shorten with each successive cell division. Shortened telomeres activate a mechanism that prevents further cell multiplication. This may be an important mechanism of ageing in tissues like bone marrow and the arterial lining where active cell division is necessary. Importantly though, mice lacking telomerase do not show a dramatically reduced lifespan, as the simplest version of this theory would predict.

Reproductive-Cell Cycle Theory

The idea that ageing is regulated by reproductive hormones that act in an antagonistic pleiotropic manner via cell cycle signalling, promoting growth and development early in life in order to achieve reproduction, but later in life, in a futile attempt to maintain reproduction, become dysregulated and drive senescence (dyosis). At the same time, castrated animals, although living somewhat longer, still experience senescence, even in the absence of reproductive hormones.

Wear-and-Tear Theory

The very general idea that changes associated with ageing are the result of chance damage that accumulates over time.

Somatic Mutation Theory

The biological theory that ageing results from damage to the genetic integrity of the body’s cells.

Error Accumulation Theory

The idea that ageing results from chance events that escape proof reading mechanisms, which gradually damages the genetic code.

Evolutionary Theories

See Theories of ageing in Senescence. These are by far the most theoretical; however, their usefulness is somewhat limited as they do not provide readily testable biochemically based interventions.

Accumulative-Waste Theory

The biological theory of ageing that points to a buildup of cells of waste products that presumably interferes with metabolism.

Autoimmune Theory

The idea that ageing results from an increase in autoantibodies that attack the body's tissues. A number of diseases associated with ageing, such as atrophic gastritis and Hashimoto's thyroiditis, are probably autoimmune in this way. While inflammation is very much evident in old mammals, even SCID mice in SPF colonies still senescence.

Ageing-Clock Theory

The theory that ageing results from a preprogrammed sequence, as in a clock, built into the operation of the nervous or endocrine system of the body. In rapidly dividing cells the shortening of the telomeres would provide just such a clock. This idea is indirect contradiction with the Evolutionary Based Theory of Aging.

Cross-Linkage Theory

The idea that ageing results from accumulation of cross-linked compounds that interfere with normal cell function.

Free-Radical Theory

The idea that free radicals (unstable and highly reactive organic molecules, also named reactive oxygen species or oxidative stress) create damage that gives rise to symptoms we recognize as ageing.

Mitohormesis

It has been known since the 1930s that restricting calories while maintaining adequate amounts of other nutrients prevents ageing across a broad range of organism. Recently, Michael Ristow has shown that this delay of ageing is due to increased formation of free radicals within the mitochondria causing a secondary induction of increased antioxidant defence capacity.^[31]

100% of dead yeast cells

100% of live cells (with the "elixir of life")

Misrepair-Accumulation Theory: This very recent novel theory by Wang et al. ^[32] suggests that ageing is the result of the accumulation of "Misrepair". Important in this theory is to distinguish among "damage" which means a newly emerging defect before any reparation has taken place, and "Misrepair" which describes the remaining defective structure AFTER (incorrect) reparation. The key points in this theory are: Misrepair does mainly not happen accidentally, it is a necessary measure of the reparation system to achieve sufficiently quick reparation of a damage. There is no damage left unrepaired. If damage was left unrepaired a life threatening condition would develop. However perfect reparation (full reconstitution) of a structure is not feasible for the reparation system in sufficient time. The solution to this dilemma, namely the exigence of sufficiently quick and sufficiently accurate reparation which evolution has favoured is a compromise: Misrepair. So the reparation system sometimes even promotes Misrepair to guarantee immediate survival of the living being. However, since Misrepair is invisible for the reparation system it accumulates in time and causes inevitably problems on the longer term: It is the actual source of ageing. Hence the appearance of Misrepair sacrifices long-term survival for the immediate survival of individual which is more important for the survival of species. Therefore the Misrepair mechanism was selected by nature due to its evolutionary advantage. Ageing is the side-effect for survival, immediate survival and species survival. Thus Misrepair might represent the mechanism by which organisms are not programmed to die but to survive, and ageing and the final individual death is just the price to be paid.

Non-biological theories

Disengagement Theory

This is the idea that separation of older people from active roles in society is normal and appropriate, and benefits both society and older individuals. Disengagement theory, first proposed by Cumming and Henry, has received considerable attention in gerontology, but has been much criticised.^[2] The original data on which Cumming and Henry based the theory were from a rather small sample of older adults in Kansas City, and from this select sample Cumming and Henry then took disengagement to be a universal theory.^[33] There are research data suggesting that the elderly who do become detached from society as those were initially reclusive individuals, and such disengagement is not purely a response to ageing.^[2]

Activity Theory

In contrast to disengagement theory, this theory implies that the more active elderly people are, the more likely they are to be satisfied with life. The view that elderly adults should maintain well-being by keeping active has had a considerable history, and since 1972, this has become to be known as activity theory.^[33] However, this theory may be just as inappropriate as disengagement for some people as the current paradigm on the psychology of ageing is that both disengagement theory and activity theory may be optimal for certain people in old age, depending on both circumstances and personality traits of the individual concerned.^[2] There are also data which query whether, as activity theory implies, greater social activity is linked with well-being in adulthood.^[33]

Selectivity Theory

mediates between Activity and Disengagement Theory, which suggests that it may benefit older people to become more active in some aspects of their lives, more disengaged in others.^[33]

Continuity Theory

The view that in ageing people are inclined to maintain, as much as they can, the same habits, personalities, and styles of life that they have developed in earlier years. Continuity theory is Atchley's theory that individuals, in later life, make adaptations to enable them to gain a sense of continuity between the past and the present, and the theory implies that this sense of continuity helps to contribute to well-being in later life.^[15] Disengagement theory, activity theory and continuity theory are social theories about ageing, though all may be products of their era rather than a valid, universal theory.

Measure of age

The age of an adult human is commonly measured in whole years since the day of birth. Fractional years or months or even weeks may be used to describe the age of children and infants for finer resolution. The time of day the birth occurred is not commonly considered.

The measure of age has historically varied from this approach in some cultures. For example, in China, Korea, and Japan, children were considered to be one year old at the moment of birth and two years old on the following New Year's day.^[34] Thus, a child could be considered two years old several days after birth.^[35]

Age in prenatal development is normally measured in gestational age, taking the last menstruation of the woman as a point of beginning. Alternatively, fertilization age, beginning from fertilization can be taken.

See also

Notes

References

• Bass, S.A. (2006). Gerontological Theory: The Search for the Holy Grail. The Gerontologist, 46, 139-144.
• Bath, P.A. (2003). Differences between older men and woman in the Self-Rated Health/ Mortality Relationship. The Gerontologist, 43 387-94
• Charles, S.T., Reynolds, C.A., & Gatz, M. (2001). Age-related differences and change in positive and negative affect over 23 years. Journal of Personality and Social Psychology, 80, 136-151.
• Fentleman, D.L., Smith, J. & Peterson, J. (1990). Successful aging in a postretirement society. In P.B. Baltes and M.M. Baltes (Eds.).Successful aging: Perspectives from the Behavioural Sciences. pp50–93
• Mather, M., & Carstensen, L. L. (2005). Aging and motivated cognition: The positivity effect in attention and memory. Trends in Cognitive Sciences 9, 496-502. PDF
• Masoro E.J. & Austad S.N.. (eds.): Handbook of the Biology of Aging, Sixth Edition. Academic Press. San Diego, CA, USA, 2006. ISBN 0-12-088387-2
• Moody, Harry R. Aging: Concepts and Controversies. 5th ed. California: Pine Forge Press, 2006.
• Rowe, J.D. & Kahn, R.L. (1987). Human aging: Usual and successful. Science, 237, 143-149
• Rowe, J.D. & Kahn, R.L.(1997). Successful aging. The Gerontologist, 37 (4) 433-40
• Strawbridge, W.J., Wallhagen, M.I. & Cohen, R.D. (2002). Successful aging and well-being: Self-rated compared with Rowe and Kahn. The Gerontologist, 42, (6)
• Zacks, R.T., Hasher, L., & Li, K.Z.H. (2000). Human memory. In F.I.M. Craik & T.A. Salthouse (Eds.), The Handbook of Aging and Cognition (pp. 293–357). Mahwah, NJ: Erlbaum.

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