Physical Development
Normal growth during adolescence includes sexual maturation and an increase in body size. The timing and speed with which these changes occur vary and are affected by both heredity and environment. Physical maturity begins at an earlier age today than it did a century ago. For example, girls have their first menstrual period at a considerably younger age than their counterparts did 100 years ago. The reason is probably improvements in nutrition, general health, and living conditions.
 See the figure Milestones in Sexual Development.
During adolescence, most boys and girls reach adult height and weight, although there is considerable variation in when this occurs. The growth spurt in boys occurs between the ages of 13 and 15½ years; a gain of 4 inches can be expected in the year of maximum growth. The growth spurt in girls occurs between the ages of 11 and 13½ years; a gain of 3½ inches can be expected in the year of maximum growth. In general, boys become heavier and taller than girls. By age 18, boys have about ¾ inch of growth remaining and girls have slightly less.
In boys, the first changes in sexual characteristics are enlargement of the scrotum and testes, followed by lengthening of the penis. Internally, the seminal vesicles and prostate gland enlarge. Next, pubic hair appears. Hair grows on the face and in the underarms about 2 years after it appears in the pubic area. The first ejaculation usually occurs between the ages of 12½ and 14, about 1 year after the penis begins to lengthen. Breast enlargement on one side or both is common in young adolescent boys and usually disappears within a year.
In the majority of girls, the first visible sign of sexual maturation is breast budding, closely followed by the growth spurt. Soon afterward, pubic and underarm hair appears. The first menstrual period generally starts about 2 years after the breasts begin to enlarge. Height increases most before menstruation begins.
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