varies from country to country and sub-jurisdiction (state, province, department, etc.) to various other sub-jurisdictions.

In my Northeastern US state, a minor (under 18) CAN be charged with "statutory rape:" the charge would be, "delinquent, to wit, 'rape of a child.'"

Now that's the law. What happens in fact may be another matter. As things get looser, look for more tolerance, and then, after a while, the law-makers (in various jurisdictions) may say, "well, they're doing it anyway....so let them do it legally.

If this is your thing, there's a large paysite, Mr.Doubles which apparently puts up lots of under-age stories. I've never subscribed, but judging by the story exceprts and what free material they have, you'll find plenty of incest, under-age, non-consensual, extreme, and pedophiliac stories.