Short answers, pulled from the story.
Females typically begin puberty at around age ten and a half; males begin at around eleven and a half to twelve. Puberty generally ends between ages fifteen and seventeen for females and sixteen to seventeen for males.
The average age of menarche in the United States is twelve and a half. Most American females experience their first period at eleven, twelve, or thirteen, though anytime between eight and sixteen is considered normal.
Puberty is initiated when the hypothalamus begins releasing pulses of GnRH, which prompts the pituitary gland to secrete LH and FSH. Those hormones signal the gonads to produce testosterone and estradiol, which drive the physical changes of puberty. LH levels increase about twenty-five-fold at the onset of puberty.
Scientific researchers have linked early puberty in females to obesity, exposure to bisphenol A (BPA), phthalates in cosmetics and plastic food containers, and certain hair care products containing estrogen. Genetic factors account for at least forty-six percent of the variation in timing among well-nourished populations, with the strongest genetic link between mothers and daughters.
Early-maturing females are at higher risk for depression, lower self-esteem, negative body image, eating disorders, and increased alcohol and drug use. Early-maturing males are more likely to show aggressive and law-breaking behaviors, while late-maturing males tend to face lower self-esteem, anxiety, and depression. Females aged fifteen to nineteen account for forty percent of anorexia nervosa cases.
The average age of puberty onset has declined since the 1840s, when the average age was fifteen for females and seventeen for males. A 2006 study in Denmark found breast development starting at an average age of nine years and ten months, a full year earlier than a 1991 Danish study. However, archaeological research suggests puberty onset today may be comparable to pre-industrial periods when severe poverty delayed later markers like menarche.