Whenever you start becoming sedentary.
It's less about age and more about exercise. You typically lose about a pound of muscle a year after the age of 40. This of course, as the answerer above noted, be offset by regular resistance exercise.
It would have to be leg and muscle mass that is the answer on a+
age and muscle mass
sarcopenia
Beginning in middle age, the answer is (generally) yes.
Human males begin getting buff when they are about the age of 14.Buffest at 24.At least that's my best guess...They also start losing brain cells at age 12.Which is really a shame because I am sure they could be very nice if they had brains at that age.Whatever though.
at age 21
Men typically lose muscle mass as they age due to a combination of factors, including a decrease in testosterone levels, reduced physical activity, and lower protein intake. These changes can lead to muscle atrophy and a decrease in muscle strength and size over time. Regular resistance training and a balanced diet can help mitigate muscle loss with age.
sarcopenia
16
Without consistent strength training, muscle size and strength decline with age. An inactive person loses 1/2 pound of muscle per year after age 20. After age 60, this rate of loss doubles. But, muscle loss is not inevitable. With regular strength training, muscle mass can be preserved throughout the lifespan, and the muscle lost can be rebuilt. Source: SparkPeople.com
Muscle mass is important in maintaining a healthy body. Depending on age and physical activity level, the average muscle mass percentage lies between 38% and 54% for men and between 28% and 39% for women.
Generally speaking an increase in muscle mass tends to be one of the later stages of puberty. Puberty can start at any age from 9 years old right up until the later teens. It usually it usually starts at around 12 or 13 years of age. Puberty lasts on average for around 6 years and muscle mass tends to increase towards the end of this period. So for a boy who started puberty aged 12 you might probably expect to see an noticable increase in muscle mass at around 16 or 17 years of age. Remember though that averages are just mathematical statistics and in the real world some boys might develop an increase in muscle mass sooner or later than the "average."