Maxu feels

[Taking 17 as the typical age of first conception this would generally mean girls in the 12-16 bracket, right?]

Yeah, that sounds about right. Young adolescent girls with pert tits, soft unused nipples and flat taut stomachs with no stretch marks would be the best females to commit yourself to. 

In fact,a lot of the reason the females of our species have evolved pert tits in their adolescent years is probably to advertise their nulliparity to the males. If you ever see some footage of a primitive tribe one of the first things you notice is that most the women have horrible saggy boobs and hard nipples from pregnancies and breast feeding. It's only the young adolescent girls that haven't been pregnant yet that have nice pert tits. This state of breast pertness is like an advertisement saying "Hey, look. I'm young and haven't had a baby yet! All my breeding years still lie ahead of me!". This is similar to the adolescent sexual swellings seen in gorilla females that disappear once they start reproducing. 

Another simple advertisement of nulliparity is a flat taut stomach with no stretch marks. I imagine this is a lot of the biological reason teenage girls like to wear short tops that show off their abdomens.