Puberty occurs much earlier in modern societies.

Relative to Victorian/Industrial age societies yes. If you compare it to pre-industrial societies it seems that the age of puberty is returning to its historical norm after a couple centuries of being unusually high.

Recently postpubescent girls and boys are children, and most people are turned off by the personalities of children (which of course makes evolutionary sense, as well as cultural sense in most societies). The age group you're thinking of is closer to 18 than to 15.

Which raises the question of why in modern societies 15-year-olds have childlike personalities. For example, in Jewish society children were traditionally considered adults at 13.