In our modern society, 10/11/12 is a young age to have children, but when life expectancy was much less, young girls were married by the age of 13 and had children around that age too. Going back down the evolutionary ladder, female animals have the capacity to reproduce from an early age because they are not expected to live long and have to fight to survive and procreate efficiently in order to ensure the survival of their species (good old Darwin). To flip the question, why do boy's balls drop *ahem* (apologies for the phrasing, i dont know the medical term) so early, when socially, they shouldnt be fathers until much later. It's all because the ideology of our society doesnt necessarily fit our biological destinies. That's why we are higher than animals - because we have the ability to escape our biological destinies and go after/create other things.
To digress even more, a major reason why women now live longer is because of the invention of reliable birth control (and its acceptance in society). In the past, women died earlier because they were constantly pregnant. Many gave birth and then became pregnant again 2 or 3 months later. My great grandma died from the strain of having 13 children in as many years for example.
In reply to 'pullmyfinger' who said that 'I would say that it is the hormones in the foods that WE eat. It's not "mother nature"' that makes girls start their periods earlier and earlier: perhaps it is that we are now having fuller diets - there is less child malnutrition
in the developed world now. Our ten year olds are generally bigger and stronger than the ten year olds of the 1600s, thats why girls are starting their periods earlier perhaps. Our diets may not be healthy because of all the processed stuff, but our bodies are growing faster because we have more generally.