<unk>  is the process of physical changes through which a child's body matures into an adult body capable of sexual reproduction. 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 sex organs. Physical growth—height and weight—accelerates in the first half of<unk>  and is completed when an adult body has been developed. Before<unk> , the external sex organs, known as primary sexual characteristics, are sex characteristics that distinguish boys and girls.<unk>  leads to sexual dimorphism through the development of the secondary sex characteristics, which further distinguish the sexes. On average, girls begin<unk>  at ages 10–11 and complete<unk>  at ages 15–17; boys generally begin<unk>  at ages 11–12 and complete<unk>  at ages 16–17. The major landmark of<unk>  for females is menarche, the onset of menstruation, which occurs on average between ages 12 and 13. For males, first ejaculation, spermarche, occurs on average at age 13. In the 21st century, the average age at which children, especially girls, reach<unk>  is lower compared to the 19th century, when it was 15 for girls and 17 for boys (with age at first periods for girls and voices break and growth spurt for boys being used as the age at onset). 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.<unk>  which starts earlier than usual is known as precocious<unk> , and<unk>  which starts later than usual is known as delayed<unk> . Notable among the morphologic changes in size, shape, composition, and functioning of the<unk> l 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 (age of maturity), the word<unk>  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<unk> .
Puberty
Postpartum period
Adolescent health
Somatopause
Adrenopause
Teenage pregnancy
Infant
Poster child