Re: At what age...
I had the talk with Tessa for the first time when she turned 7. That was because she was starting to develop breasts. We have the talk every year on her birthday (she is 9 1/2 now) and whenever she wants to talk about it.
I choose to tell her at 7 because she was starting to show signs of developing, and didn't want her to get her info from friends, or be caught by surprise. Thankfully... she still hasn't started to develop hair (as far as I know anyway... ) and hasn't started her period yet.
My friend just now told her daughter on her 10th birthday... Her daughter shows no signs of developing... she just told her because she's 10. I think you kind of have to go with your daughter's maturity level and development... If she isn't showing signs of developing and she's younger, I would let it go. But you say she needs to wear a bra already... So it might be time to talk. When I was researching puberty in girls it said that from the time they actually develop breasts (if they do...) it can come as quickly as one year or as long as 4 so it's a crap shoot... Good luck