You're supposed to wear bras when you get boobs, and even then you start off with training bras until they grow big enough to fit into a cup size. Some girls do start wearing bras earlier than they need to for aesthetic reasons (they want to seem cooler, older, hotter). I do feel that 6 and 7 is way to young, because their boobs aren't even beginning to come in yet. Breasts only start to develop around puberty, with the earliest age being 8 and the oldest being 13. Some girls do try wearing them before they need to though, just to seem cooler, prettier, and older. I had a few friends back in grade 3 who thought they were the shit because they owned a bra. I even thought they were cooler, but now that I look back, they just looked ridiculous because their bras were way too big for their chest lmao.
I was 11 when I started wearing one. They were padded like a ton haha When puberty happens foe the little girl is when she should wear one. The exact age depends and not all girls bloom early or quickly
It has less to do with age and more to do with where the girl is in terms of puberty. But to be honest, there's no harm in letting your daughter wear a bra earlier than it's needed if it makes her feel more comfortable.