"The age of menarche, a girl's first period, has dropped dramatically over the past century, from about 14 or 15 in 1900 to about 11 or 12 now."
http://www.smh.com.au/articles/2003/03/28/1048653855313.html
Actually, there is an explanation for it. The cause is the modern day diet, which is very rich in saturated fats. This kind of fat speeds up sexual maturation and the ageing process. The more saturated fat you eat, the more testosterone you get - and estradiol and progesterone in the case of women. While this might be good for gaining muscle, it's bad for your health and longevity, because it speeds up oxidative stress and decreases your levels of superoxide dsismutase, glutathione and several enzymes related to the longevity.
The reason why saturated fats cause this is because the Human body was designed to survive and reproduce. When calories are low, your body switches into a survival mode, which speeds down metabolism and decreases the levels of sexual hormones - you can't spend energy reproducing if you're trying to survive. Conversely, when calories are abundant, the hypothalamus in the brain signals the release of luteinizing hormone, which travels to the testes and ovaries and orders them to produce more sexual hormones. The general rule is this: nature makes you live so you can reproduce, and the sooner you reproduce, the sooner you can age and die.
The modern diet, especially the steretypical American diet, which is characterized by an abundance of fatty treats, which almost never contain monoinsaturated fats by rather only saturated, results in early sexual maturation, accelarated ageing and death. The early sexual maturity of American girls is the result of the American diet of fast food. In Southern European, where people still have wholesome foods as the base of the diet, girls have their menarch much latter, and people age much slower. Hope this helps.
SUCKMYMUSCLE
