As you grow, there is a cell in your body when you're a baby. As the cell multiplies into more cells, you grow. The more cells there are, the more complex that organism is. Cells are continually multiplying and dying off, to be replaced by new cells. How much we grow is decided by our hormones and our genetic makeup, that is, what our parents were like. For example, tall parents are more likely to have children who also grow into tall adults. People do actually shrink when they get older. The discs between the vertebra in a person's spine compress together over time, and this makes people actually shrink slightly with age. Osteoporosis also causes people to shrink with age, as it causes bones to break down, and new bone growth doesn't take place as the bones are missing vital components to help them regenerate.