Apr 19, - It is difficult to know exactly when your penis will stop growing. During puberty, a young man's testicles begin to grow larger, followed by the penis. On average the penis will reach its final size about years after the testicles enlarge. Most guys are fully developed by the time they are years old. Mar 23, - The penis typically reaches its final size 4 to 6 years after the testicles enlarge. On average, young men will experience the most rapid growth of their penis and testicles between the ages of years.