Puberty is the term used to describe when a child starts on the process to become an adult. It is triggered by an increase of human growth hormone in the pituitary gland, which in turn increases the production of estrogen and testosterone. In females, it is characterized by the beginning of menstruation. There is no single event that defines puberty in males. However, both sexes start showing signs of development:

- Secondary sexual characteristics. Pubic hair in both sexes. Facial hair and increased musculature in males. Development of the breasts and hips in females.

- Extremely fast growth. Children going through puberty can grow 30-60 centimeters in height in periods as short as twelve months, although the range and rate of growth varies widely.

- In males, sudden deepening of the voice and increased penis size and development of testicles.

Studies of persons going through puberty also show that brain development changes, and sleep patterns start to change so that teenagers tend to want to stay up later at night. These changes in hormones and brain development are thought to account for much of the behavior of teenagers, particularly a difficulty in controlling impulses.

Precocious puberty describes a condition where a child starts puberty much earlier that usual. Although this can be the result of environmental exposure to hormones (see Act Your Age), the age of puberty has been decreasing for both sexes over the past forty years.

Puberty at Wikipedia