Puberty is the period of human development during which physical growth and sexual maturation occurs.

Beginning as early as age eight in girls—and two years later, on average, in boys--the hypothalamus (part of the brain) signals hormonal change that stimulates the pituitary. In turn, the pituitary releases its own hormones called gonadotrophins that stimulate the gonads and adrenals. From these glands comes a flood of sex hormones androgens and testosterone in the male, estrogens and progestins in the female--that regulate the growth and function of the sex organs. It is interesting to note that the gonadotrophins are the same for males and females, but the sex hormones they induce are different.

In the United States, the first sign of puberty occurs on average at age 11 in girls, with menstruation and fertility following about two years later. Boys lag behind by about two years. Puberty may not...