When guys hit puberty, a number of changes occur in the body including increased hair growth on certain parts of the body (chest, pubic area, legs, underarms) and a deepening of the voice. Puberty will also cause an increase in the size of a young man’s testicles, followed by growth in the size of the penis. 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 12-16 years.