We have the opposite problem. My daughter is 11 and tall and "well developed" in fact I don't think I had breasts that size until after I had my first child. However, she has developmental delays and a lot of issues around how things feel and she refuses to wear a bra. It's winter right now but spring and summer are coming and her breasts just keep getting bigger:( The problem is compounded by the fact that I am a bad role model. I absolutely hate bras and don't even own one. I wear a couple of layers in the summer with sports tops and apart from the gravity years and multiple pregnancies have brought I don't need to wear a bra.
It is so culturally determined. Why should she have to wear a bra? My daughter is comfortable without one and doesn't do any sports where she would uncomfortable without one. She does ballet and even the older girls don't wear bras (or underwear) under their dance clothes. The girls with big breasts just wear several tight layers, works fine.
This is a topic that I struggle with all the time just now. Don't even ask what it is like when a girl with developmental delays starts menstruating right after she turns 11.