My daughter is 14 and she started changing around 10 and begin her periods at 12.
Her dentist and pedi kept a watchful eye on her because she by 10 she had lost all the teeth she was supposed to lose AND already had her 12 yr molars. Her mouth is perfect and we were told that she might have early puberty because she started losing teeth at 4. Somehow that is related, I don't know how or why.
Also, my daughter is very active in athletics (cheer, black belt, etc) and the pedi said more active girls usually don't hit puberty too early.
She has some friends were were not as trim or active as she has been and they all started earlier. There is not an ounce of fat on my daughter....she maintains an excellent diet. I have one of those kids who do not like fast food, etc. She prefers fruits, veggies, salads, etc.
Good luck.. They grow up WAY too fast!