If you were born a few thousand years ago it may be completely reasonable to sleep with a kid.

That simply is not correct. While kids have been having sex with kids since, like, forever, looking back to medieval Europe, or even Rome, it was not normal for adults to be having sex with children, especially prepubescent children. Marriage ages for people that weren’t nobility (e.g., getting married off to solidify political alliances) were typically early 20s. Children getting pregnant has always been a very dangerous proposition, since it’s much more likely to lead to maternal mortality.

Part of the confusion about all of this is that we think people used to live compressed lives, since we see average lifespans in the 40s. But that’s not taking into account the ferocious infant mortality rates; if you survived into your late teens, you were probably going to make it to your late 60s/early 70s, even in the middle ages. We think that, since ‘average’ life spans were 40-odd years, that people must have been marrying very young, but the evidence doesn’t really bear that out.