I didn't get mine until I was 15...I wasn't athletic, I was just a late bloomer.
I don't think my mom got hers until she was 13-14. IMO, if your daughter doesn't know about her period yet, you need to talk to her. You don't want her to start and not know what is going on. I'd talk to her doctor or perhaps a counselor at school about the best way to approach this, because I have no advice to give. If it weren't for my maturation class in 5th grade, I'd never have known what a period was.
As far as maturing earlier, I very much believe it's partially due to growth hormones in meat and milk and higher levels of body fat than in previous generations. But that's another soapbox for another day
