For males, first ejaculation, spermarche, occurs on average at age 13. The major landmark of puberty for females is menarche, the onset of menstruation, which occurs on average between ages 12 and 13. On average, girls begin puberty at ages 10–11 and complete puberty at ages 15–17 boys generally begin puberty at ages 11–12 and complete puberty at ages 16–17. Puberty leads to sexual dimorphism through the development of the secondary sex characteristics, which further distinguish the sexes. Before puberty, the external sex organs, known as primary sexual characteristics, are sex characteristics that distinguish boys and girls. Physical growth-height and weight-accelerates in the first half of puberty and is completed when an adult body has been developed. 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. It is initiated by hormonal signals from the brain to the gonads: the ovaries in a girl, the testes in a boy. Puberty is the process of physical changes through which a child's body matures into an adult body capable of sexual reproduction.
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