Mine started to grow when I was nine too. I wasn't even overweight, I was hella skinny and for a year only one of them was growing so I pretty much had one boob lol.
Oh and Kellie, the being active part can't be true, when I was 10 - 11 I did a lot of dancing (like 5 - 6 days a week) and it didn't keep me from bleeding lol.