Puberty happens when it happens. Is her development visible to others when she's got clothes on? If it is, then a bra is an appropriate step. If she's worn undershirts or tank tops, a sports bra style may be a more comfortable transition for her.
Talk to her, and ask if she's comfortable with wearing one, and hopefully she'll feel she's ready. This is a big step, it's part of the transition to becoming a woman, and should be celebrated. Make an event of bra shopping, go some place "grownup" for lunch, maybe take her to get her nails polished at your salon. This is the perfect time for you to set the tone for how she feels about her body changing, and making it something special will help her to feel good about it.
Oh, and another little change that always seems to go with this one: antiperspirant/deoderant. Hormone production that leads to the onset of puberty also leads to the onset of odor. Get her to start wearing a deoderant BEFORE someone notices or complains that she needs one. That is very embarassing, and happened to me in elem school. My Mom thought I was too young, and didn't let me wear it, but I really needed it. It wasn't until my teacher called and said something to her that she changed her mind. Of course, she told me the teacher called, and told her I smelled, so now I needed to use this new stuff. I was humiliated, didn't want to have to go back to school, and felt generally miserable about the whole thing. A nice, teen formula with a baby powder scent would be a good thing for her to start incorporating into her daily grooming routine, and too early is WAAY better than too late.
Jess