In boys, the testes, prostate, seminal vesicles, and penis mature, and facial, pubic, and underarm hair grows. Normally, these changes occur sequentially during puberty, resulting in sexual maturity. The interval from breast budding to the first menstrual period generally is about 2 years. The girl's body shape changes, and the percentage of body fat increases. The growth spurt accompanying puberty typically begins even before the breasts start to develop.
Growth is fastest relatively early in puberty, before menstrual periods begin.
Then growth slows considerably, usually stopping between ages 14 and 16. In contrast, boys grow fastest between ages 13 and 17 and can continue to grow into their early 20s. The age at which puberty begins seems to be influenced by a child's general health and nutrition as well as by socioeconomic and hereditary factors. In Western Europe, the average age at which a girl has her first menstrual period decreased by 4 months for each decade between 1850 and 1950, but the age hasn't decreased in the last four decades. Periods also start earlier among girls who live in urban areas and among those whose mothers started menstruating early.