There’s no need for one right now, as long as she’s comfortable. But given the sensitivity of early breast tissue, some girls find it more comfortable to wear a soft, gently supportive undergarment like an undershirt or sports bra. Let her decide. Girls’ feelings about their first bra are decidedly mixed. Some are thrilled to take this early step toward womanhood, but others are mortified by the thought of wearing a bra to school.
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.
Physical development starts at different times and progresses at different rates in different people. So, the beginning of the development that comes with puberty varies from person to person — and that's completely normal.
The earliest physical change of puberty for girls is usually breast development, which most often begins around 10 or 11 years. But it's perfectly normal for breast development to start anytime between the ages of 7 and 13. A girl's first menstrual period usually occurs about 2 to 2½ years after breast development begins.