I don't think people should downvote without explaining, but the suggestion of having them sleep in separate rooms is a little ridiculous if they're already having sex.
Trust me, as someone who's parents did the same thing, the only thing it will accomplish is pissing everyone off (the teens because they're being treated like kids, the parents because they suspect something is happening regardless of what is). Either give them "the talk" and let them make decisions like adults, or decide they are still children and don't let her stay over.
It's a pretty wide range, I'm included at 27 and I've got a couple friends younger than I am who'd probably get it too.