12-13 is about average...younger than 10 is definitely out of the ordinary. Some blame the hormones in our food, others say it's a mass shift because women as a whole are menstruating for longer than ever these days. We start earlier, delay childbearing and hit menopause later. And since we women have the 'wonderful' tendency to sync cycles...it would stand to reason that we're affecting the younger females around us more than ever, in addition to outside environmental factors.