Do you remember being 13? Maybe my experiences/generation was different, but in my class (there was about a thousand of us.) there were plenty of 13 year olds having sex (with other 13 years old and not that much older) and even a couple pregnancies that were carried to term.
The marriage/mating drive is about babies, not "wow my penis finally works, well I guess it's just one vagina for me until I die."
The parents force a telepathic bond between 7 year olds that kicks in when the parents think the time is appropriate, it's like the Manchurian candidate with sex.
If it wasn't for the the telepathic bond, the Vulcan in the heat, boy or girl, it would just be a question of proximity, and you'd end up with weird mutt pairings of princes and hillbillies, which high society is pursed to frown upon.