Throughout history men have fallen for teens with fully- or near-fully-developed breasts. Biologically, it's a sign of early fertility. Assuming you are much older, marrying such a person will give you more fertile years than marrying someone your own age. Also, if you are over 40, marrying such a person is more likely to result in fewer birth defects than having a child with a 40 year old wife.

As for people who are attracted to prepubescent girls, I don't have a biological explaination, but I'm betting there is one. Further research is needed.