Hair usually begins to grow on various parts of the body when a boy is between ages 11 and 14 years. Hair can continue to spread to other parts of the body until a young man reaches about age 20.

Sparse growth of slightly pigmented pubic hair at base of penis around age 12.4 -- Adult-type hair with spread to the inside of thighs but not up the abdomen (15.3 years).

Hair is typically seen on a young man's face, underarm area, pubic area, abdomen, chest, arms, legs, and buttocks. The amount and distribution of hair can vary considerably from one man to the next -- this is entirely normal and may have genetic tendencies.

Increased growth of the penis and scrotum often starts at about age 13 and continues until adult size is reached about 2 years later. Keep in mind, there is a fair degree of variation in the age for genital development from one boy to the next. See 'Penis Size' for more on this.

Thinning and reddening of scrotum occurs around age 12 years.

While both girls and boys will begin a rapid growth spurt sometime between the ages of 10 and 15, the growth spurt in boys usually occurs about two years after the growth spurt takes place in girls of the same age.

The body takes on a new more muscular and angular shape in boys under the influence of testosterone. This generally begins around age 12.5 when testosterone causes muscle mass to increase. The greatest effect can usually be seen in the upper chest and shoulder muscles. Testosterone also causes bones to lengthen, giving young men a heavier bone structure and longer arms and legs.