Puberty is the process of physical changes by which a child's body matures into an adult body capable of sexual reproduction to enable fertilisation. It is initiated by hormonal signals from the brain to the gonads; the ovaries in a girl, the testes in a boy. In response to the signals, the gonads produce hormones that stimulate libido and the growth, function, and transformation of the brain, bones, muscle, blood, skin, hair, breasts, and sexual organs. Physical growth—height and weight—accelerates in the first half of puberty and is completed when the child has developed an adult body. Until the maturation of their reproductive capabilities, the pre-pubertal, physical differences between boys and girls are the genitalia, the penis and the vagina.
On average, girls begin puberty at ages 10–11; boys at ages 11–12. Girls usually complete puberty by ages 15–17, while boys usually complete puberty by ages 16–17. The major landmark of puberty for females is menarche, the onset of menstruation, which occurs on average between ages 12–13; for males, it is the first ejaculation, which occurs on average at age 13. In the 21st century, the average age at which children, especially girls, reach puberty is lower compared to the 19th century, when it was 15 for girls and 16 for boys. This can be due to any number of factors, including improved nutrition resulting in rapid body growth, increased weight and fat deposition, or exposure to endocrine disruptors such as xenoestrogens, which can at times be due to food consumption or other environmental factors. Puberty which starts earlier than usual is known as precocious puberty. Puberty which starts later than usual is known as delayed puberty.
Notable among the morphologic changes in size, shape, composition, and functioning of the pubertal body, is the development of secondary sex characteristics, the "filling in" of the child's body; from girl to woman, from boy to man. Derived from the Latin puberatum (age of maturity), the word puberty describes the physical changes to sexual maturation, not the psychosocial and cultural maturation denoted by the term "adolescent development" in Western culture, wherein adolescence is the period of mental transition from childhood to adulthood, which overlaps much of the body's period of puberty.
Two of the most significant differences between puberty in girls and puberty in boys are the age at which it begins, and the major sex steroids involved.
Although there is a wide range of normal ages, girls typically begin the process of puberty at age 10 or 11; boys at ages 11–12. Girls usually complete puberty by ages 15–17, while boys usually complete puberty by ages 16–17. Any increase in height beyond the post-pubertal age is uncommon. Girls attain reproductive maturity about 4 years after the first physical changes of puberty appear. In contrast, boys accelerate more slowly but continue to grow for about 6 years after the first visible pubertal changes.