It depends if she is ready to wear one. Some girls develop faster and others develop slowly. I had a friend who started wearing one when she was nine. I was eleven when I started wearing one. It should be based on if your daughter is developed enough to wear one. Most girls start out with a first bra.
Once puberty hits & things begin to change it can be quick. It'll give her support but more importantly she'll know that the change is normal & that she can come to you for help & support (ha ha) when the changes occur, it can be scarey, but not when help is there-you go be a parent good for you