Final answer:
Puberty in girls is triggered by the hypothalamus and pituitary gland, which release hormones that stimulate the ovaries to produce estrogen. The onset of puberty can be influenced by factors such as nutrition, genetics, environment, and psychological stress. Body fat and the hormone leptin play a role in determining the timing of menarche.
Step-by-step explanation:
Puberty in girls starts when the hypothalamus "tells" the pituitary gland to secrete hormones that target the ovaries. Two pituitary hormones are involved: luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These hormones stimulate the ovary to produce estrogen. Estrogen, in turn, promotes growth and other physical changes of puberty. It stimulates growth and development of the internal reproductive organs, breasts, and pubic hair.
In addition to age, multiple factors can affect the age of onset of puberty, including genetics, environment, and psychological stress. One important influence may be nutrition. Some studies indicate a link between puberty onset and the amount of stored fat in an individual. Body fat, corresponding with secretion of the hormone leptin by adipose cells, appears to have a strong role in determining menarche.
The rush of hormonal activity at the beginning of puberty seems to depend on reaching a critical amount of body fat necessary for successful reproduction, at least in girls.