This is a frequent debate in both the erotica/erotic romance quarter and the young adult quarter.
Yes, in real life, teens have sex. Sometimes even preteens have sex. Sometimes it's with a partner of a similar age; sometimes it's with an adult partner, which in real life is almost certainly going to be statutory rape at the very least. But it happens. (I knew a guy who at age 20 was approached by a 13-year-old--whom he'd never met before--for sex.)
In real life, back in the day, people were often married before they were 18, particularly girls. In arranged marriages today, that still sometimes happens.
In real life, teens might have threesomes or swap partners, or experiment with kinks. They pretty much certainly masturbate. They might date more than one person at the same time, whether or not sex is involved; I know that personally, I was aware at about age 12 or 13 that the whole "one person at a time" dating and marriage concept made absolutely zero sense to me. I wasn't thinking in terms of sex at that age, but just didn't see why someone had to be in a monogamous relationship--or cheat on their partner--if they loved more than one person.
FICTION IS NOT REAL LIFE. Most publishers don't want to risk censorship or legal action for portraying extremely explicit, graphic sexual situations with participants under 18. Polyamory is a fine line to draw if it *doesn't* involve sex; one of my YA publishers recently published a book about three boys who form a romantic relationship, but I'm not sure how much if any sexual content it contains. I do know this publisher is very, very careful about depictions of sexual activity in their books, because their goal is to have their books--which are all about GLBTQetc. characters--in school libraries so kids on those spectrums can find them.
Ironically, as I mentioned in a thread about threesomes in the YA subforum here, the head editor of that same publisher told me they would *not* consider a novel I was plotting about a polyamorous relationship among teens; however, based on comments the editor made, I think she was incorrectly conflating polyamory with polygamy of the Warren Jeffs flavor.
But anyway, the point is that what real-life teens do (or did, in the case of historical fiction), and the real-life ages of consent in various states and countries, is pretty much irrelevant to what *publishers* will accept in the *fiction* they put out. Publishers are constrained by what readers and the law will accept, and they'd rather stay far, far on the safe side of that line.