Answer:
Well it depends. Usually around 9. If you don't have anything of a bust just yet, wear a training bra because if you don't your breasts can begin to grow irregularly. But if your breasts are already starting to grow, ask a female parent or visit a bra shop, and they can measure you and fit you with a bra, a process that is not very invasive. It's normal to feel embarrassed about things like this, but try not to be, it's all a part of growing up as a female in our culture.