If you ever see some footage of primitive tribes that don’t wear clothes one of the first things you notice is that the adult women have horrible saggy boobs. This is what adult boobs are supposed to look like. A girl’s boobs are at their pertest and most attractive in her adolescent years before she starts reproducing. Once she starts having babies and breastfeeding, usually by the late teens, her boobs quickly lose their adolescent pertness and become less attractive. The pert boobs of the average nulliparous (hasn’t had a baby yet) 14yo girl are much more attractive than the drooping boobs of the average 20yo who’s had a baby or two.
The 14yo girl’s boobs are an honest advertisement of nulliparity. They’re more of an advertisement of future fertility than current fertility. They’re a signal saying “I’m approaching reproductive age and ready to start mating. I haven’t started reproducing yet and have all my fertile years remaining. Acquire me at this age and you can have all my fertile years to yourself and I’ll start giving you offspring soon.” Men’s preference for pert adolescent boobs has co-evolved with this signalling.
Another honest advertisement of nulliparity is a tight midriff that hasn’t been stretched by pregnancy. This is presumably the reason adolescent girls like to wear short tops that show off their stomach