Girls usually start developing Breasts from age 10. However, it can vary from age 7 to age 13. The Age girls stop developing breasts depends on the girl.
Some develop breasts up until age 16 but some can grow upto in their early twenties. Some girls reported not having developed breasts until age 19 and went from cup size A to C from 19 to 21.
So, it all depends on girl to girl.