Sometime between ages 9 and 15. The first noticeable change occurs when the testicles begin to grow, the scrotum becomes redder and coarser, and a few pubic hairs appear at the base of the penis. Body shape changes as the hormone testosterone causes muscle mass to develop and height increases. Sometime between ages 11 and 16. The testicles and scrotum continue to grow and the penis becomes longer.
Pubic hair becomes both coarser and more curled and spreads to cover the area between the legs.
The body increases in height, the shoulders broaden and the hips narrow. The voice begins to deepen as the voice box or larynx becomes larger. Sparse facial and underarm hair appear. Sometime between ages 11 and 17. The testicles continue to increase in size and the penis begins to increase in circumference as well as in length.