Dd1 is 13.5. She was quite an early bloomer and started developing at 10 or so, first period at 11 and a quarter. I bought her her first bras around this age at my suggestion but she seemed to accept she needed one. She wouldn't let the assistant fit her though but was happy to let me come into the changing room with her with a variety of sizes.
Since then I've tried to keep on track with buying her new ones and bigger sizes but its becoming more and more taboo with her. It's got to the point now where I can't even mention anything about bras without her clamming up or changing the subject.
A few months ago she asked me if she could wear a minimiser bra. I was surprised but said I didnt know if they came in her size. She's not huge btw and her figure is in proportion imo. But she dances a lot so maybe she's noticed a lot of dancers are quite flat chested. Anyway we both sort of forgot about it and she hasn't mentioned it again.
Even though she doesn't let me see her undressed and I completely respect that, I had a feeling she could probably do with a bigger size. So the other day I bought her some new bras one cup size bigger and left them on her bed to try in privacy. So she's stuffed them in her cupboard untried.
I do worry a lot about her as she's a very closed book generally and I often can't tell what's going through her mind. I want to balance being there for her and listening to her but also respecting her privacy. But sometimes I just don't know how to talk to her as she just clams up and I feel like I'm walking on eggshells. It seems like she doesn't like her new body but looking back, I didn't want boobs at 13 either.