A girl's breasts will begin to develop generally between the ages of 8 and 11 when her breasts begin to bud. There are five stages of breast development in all, but your tween will only experience a few of these stages before reaching adulthood. At times like these girls often try to hide their breast development under baggy clothes, but that can also encourage taunting and teasing.
There are several things you can do to help your daughter through this change. For starters, talk to her about why her body is changing and that this phase of development is normal for girls her age. Point out that all her girlfriends will also go through these changes, and probably sooner rather than later.
If your daughter is more developed physically than other girls her age, it might be a good idea to have her fitted for a bra. Most major department stores offer fittings for free, and they are a good way to make sure that your daughter is wearing a bra that's right for her. If she's too embarrassed to be fitted, you might want to purchase several bras in different sizes in order to find the size that's best for her.
Also, take the time to point out that the girls who are teasing her are doing it to either make themselves look "cool" in front of their friends, or because they are jealous that they haven't begun to develop. Boys who tease her may be doing so to get her attention, or to propel themselves to the top of the social ladder.
Arm your daughter with coping techniques so that when the teasing begins, she'll be better able to manage it. For example, teach her to ignore the comments her classmates make, because if she doesn't react they'll be less likely to continue to tease her. Another possible deterrent to teasing is to make a joke out of it. When someone comments on how her breasts have begun to develop, she could remark, "No kidding, I never would have known, thanks for the tip."
If the teasing takes an ugly turn or doesn't stop, it might be time to contact your child's guidance counselor to ask for help or to intervene.
As tweens begin to change physically it's important for them to understand that your support and love is still there for them. Be sure you take the time to let your daughter know that you're proud of her and her accomplishments, and that you're excited to see her change and grow into a responsible teen.