Like everything in nature, your penis goes through a series of changes over your lifetime. Each phase is controlled mostly by your testosterone levels.
Somewhere between the ages of 9 and 15, your pituitary gland releases hormones that tell your body to start making testosterone. Puberty begins and brings changes. Your testes (testicles), scrotum, penis, and pubic hair all begin to grow. Testosterone levels peak in your late teens to early 20s.