Adolescents (10–20 years)
Adolescence occupies the years between childhood and adulthood, starting at around 10–11 years of age for girls and 12–13 years for boys and ending at around 18–20 years for both sexes. The start of adolescence is related more to total body weight than to age and seems to be initiated when the weight reaches about 30 kg (66 lb), at which point there is a very marked increase in height followed in six months to one year by a marked gain in weight.This is accompanied by the changes needed for development of sexual maturity. The accumulation of body fat seems to be critical for girls and if this is less than about 22 per cent when the weight reaches about 46 kg there is likely to be delay in the onset of menstruation (menarche). The rapid increase in height and weight lasts 3–4 years and is followed by a period of slower growth. The general body changes are usually over by about 20 years of age but bones go on getting heavier for another 3–4 years provided nutrition is good and there is regular exercise, especially lifting and carrying. This period of continued bone growth requires much dietary calcium and is particularly important for women because the more calcium they lay down in their bones during these years the less likely are they to suffer from post-menopausal osteoporosis.