Welcome to the wonderful world of the Pituitary Gland!
The pituitary is often called the “Master Gland”, because it either makes or triggers the production of a huge range of human hormones. In particular, things like Luteinizing hormone and Follicle stimulating hormone which trigger ovulation and menstruation in women, and Thyroid Stimulating hormone and Growth hormone, which regulate skeletal growth and metabolism. The pituitary also releases melanocyte simulating hormones, which affect sex drive and appetite, as well as skin color. Um…what’d I leave out? Oh, testosterone, of course - while it’s mostly produced in the testes, it’s triggered by hormones released by the pituitary and the hypothalamus.
So what triggers the pituitary to do its thing and shift the hormonal cocktail at certain ages to create certain effects? Er…um…well…that’s the part we’re not so sure about.
One theory (not proven, but a theory) of *female *reproductive maturity is that a certain level, or perhaps ratio, of estrogen must be present in her body before her pituitary starts pumping out “puberty now!” hormones. To anthropomorphize a bit: Nature can “tell” when a girl is healthy and large enough to start having babies when she’s got some fat on her bones. Estrogen is stored in fat. So if she’s got enough estrogen, she’s probably fat enough to start having babies without dying.
Obviously, there’s something else missing here, because most obese toddlers don’t start puberty. And some kids who do have early puberty don’t have excessive body fat. But it’s a theory, and it hasn’t been discarded, and it may help to explain why our (well-fed) daughters are hitting puberty a bit sooner than their great-grandmothers did.