Girls have been menstruating earlier each generation. This generation, it has a lot do with hormones and whatnot in the water and food supply. Better foods, higher body fat percentages, etc. all influence menarche as well.
I started menstruating when I was 11, I think. I'm pretty sure my mom started when she was 13 or 14. My sister I think was 12. A family friend of ours, 2 years younger than my sister, started when she was 9. It was really unexpected and her doctors were concerned, but she turned out to be fine.
I don't think your daughter's case is unusual, but you may want to talk to her school counselor(s) and see if they have any suggestions regarding introducing her to the subjects of reproduction, menstruation, and taking care of herself. I remember having that discussion for the first time in 3rd grade, I'm not sure when they do that sort of stuff in school now. You'll probably want to find out.
Good luck!
