especially if girls don't wear undershirts, they should start wearing a bra right around when puberty starts, even if they haven't filled out yet. for one thing, it helps get used to feeling it. for another, areola and nipple can be visible through the shirt, or seen through sleeves and the neck collar, even if not baggy or loose, if some situations move her in such a way that a person, intentionally or not, can catch a glimpse. Many girls would find this quite embarrassing. a bra keeps anything from being seen.
My mom forced some kind of sports bra wannabes on me when I was in 4th grade (I always "forgot" them at school when we had sports class 😂) and in 5th grade she gave me one of her bras to try on. They were the right size so she had bought me 2 to have.
My mother noticed that I was beginning to develop breasts and the last straw was when my nipples started showing from beneath my shirt. Since I was only 8 years old when I started developing, I started out with sports bras.
6th grade a girl rubbed my back looking for a bra and was baffled not to find one. Kinda creeped me out. She tried to make me feel bad about not wearing one, but I was such a late bloomer I still only had nipples... like no tissue
7th grade I got insecure because everyone else was wearing a bra but I wasn't so I would wear one every other day. By the eight grade I wore one everyday.