I wonder if there isn't a link to the focus on "dating" earlier. If you're supposed to be mixing it up with the opposite gender fairly early, maybe your body wants to be ready.The interesting thing, JL, about menstruation is that women in the western world menstruate far more frequently than women ever have before. We start earlier, end later and do it more often (because we have babies less.) Scientists are currently unclear as to the implications of this.. but, it seems to me that less isn't inherently a bad thing.
(In fact, when I went to taking the pill in such a way as to reduce the number of periods I have in a year, the doctor went on and on about how having less periods isn't going to hurt me, physiologically, but that many women object to it because society tells us that women have periods and you are somehow less of one if you choose not to.)
No evidence to support this, just a random idea.