Physical Development: Age 12–19

Adolescence—the transition period between childhood and adulthood—encompasses ages 12 to 19. It is a time of tremendous change and discovery. During these years, physical, emotional, and intellectual growth occurs at a dizzying speed, challenging the teenager to adjust to a new body, social identity, and expanding world view.

Perhaps no aspect of adolescence is as noticeable as the physical changes that teenagers experience. Within the span of a few years, a dependent child becomes an independent and contributing adult member of society. The start of adolescence also marks the beginning of Freud's final stage of psychosexual development, the genital stage, which pertains to both adolescence and adulthood.

Puberty is the time of rapid physical development, signaling the end of childhood and the beginning of sexual maturity. Although puberty may begin at different times for different people, by its completion girls and boys without any developmental problems will be structurally and hormonally prepared for sexual reproduction. The speed at which adolescents sexually mature varies; the beginning of puberty in both genders falls within a range of 6 to 7 years. In any grouping of 14‐year‐olds, for example, one is likely to see teenagers in assorted stages of development—some appearing as older children and others as fully mature adolescents. Eventually, though, everyone catches up.

Hormones are responsible for the development of both primary sex characteristics (structures directly responsible for reproduction) and secondary sex characteristics (structures indirectly responsible for reproduction). Examples of primary sex characteristics are the penis in boys and the uterus in females. An example of secondary sex characteristics is the growth of pubic hair in both genders.

During childhood, males and females produce roughly equal amounts of male (androgen) and female (estrogen) hormones. At the onset of puberty, the pituitary gland stimulates hormonal changes throughout the body, including in the adrenal, endocrine, and sexual glands. The timing of puberty seems to result from a combination of genetic, environmental, and health factors.

An early sign of maturation is the adolescent growth spurt, or a noticeable increase in height and weight. The female growth spurt usually begins between ages 10 and 14, and ends by age 16. The male growth spurt usually begins between ages 10 and 16, and ends by age 18.

Girls generally begin puberty a few years earlier than boys, somewhere around ages 11 to 12. Increasing levels of estrogen trigger the onset of puberty in girls. They grow taller; their hips widen; their breasts become rounder and larger; hair grows on the legs, under the arms, and around the genitals; the labia thicken; the clitoris elongates; and the uterus enlarges. Around the age of 12 or 13, most girls today begin menstruating, or having menstrual periods and flow. The onset of menstruation is termed menarche. At this time, females can become pregnant.

Increasing levels of the hormone testosterone trigger the onset of puberty in boys around ages 12 to 14. Boys become taller, heavier, and stronger; their voices deepen; their shoulders broaden; hair grows under the arms, on the face, around the genitals, and on other parts of the body; the testes produce sperm; and the penis and other reproductive organs enlarge. At this time, boys can impregnate sexually mature girls. Teenage boys may also experience the harmless release of semen during sleep, termed nocturnal emissions (wet dreams).

The resulting changes of puberty can have wide‐ranging effects on teenagers' bodies. For both adolescent girls and boys, differences in height and weight, general awkwardness, emotional ups‐and‐downs, and skin problems ( acne vulgaris, or pimples) are common. These and other changes, including the timing of sexual maturation, can be sources of great anxiety and frustration for the blossoming youth.