Dd1 is 13, Year 9. She started wearing bras aged 11 or so but was very shy about the whole thing, didn't want to be measured or get any sort of assistance in shops. I took her to M and S and got her to try lots of different sizes and we chose based on what I thought fitted her and what felt comfortable.
When she'd outgrown those, we went to Debenhams and she reluctantly let me measure her in the cubicle and I then discussed with the assistant what size she might need. She was a 30D which I think is right based on all the bra intervention threads!
So she's been wearing 30Ds for a while but how do I know when she needs to be refitted? She's very shy and if I ask if they fit ok I just get a 'yes, they're fine' sort of thing.