So, there's no underage sex allowed on the site, and I both understand and agree with that ruling. Is it allowed, however, for a character (who is a villain, btw) to boast that they COULD have had sex with another character back when they were underage and would've gotten away with it if they had so desired? Basically to demonstrate how much influence they believe themselves to have, how much they think they can get away with, and more or less how sick they are or could be without actually doing the act in question? If it's not okay, it's fine, I'll come up with something else. It's just a line that struck me while writing and I wanted to be sure about it before I actually posted it.
I find it as a general rule is when talking about the past is don't mention ages. It would make your villain look more creepy then evil. TV Tropes has a whole section on Even Evil has Standards. A good villain is someone who has a point and does bad or terrible things but for what they think are the right reasons.
If it is done in a reporting, and non-arousing manner, it can be done. Though instead of the character being an evil villain, they might just serve the purpose of an asshole.