Its a hard one to cover in the current climate. Underage sex, even between two minors (which, to be honest, is how most people lose thier virginity and many of the moral majority seem to forget this) is sometimes seen as BAD because it is paedophillia and there have even been some prosecutions of teenagers who had consensual sex with other teenagers under the underage sex legislation. It is this kneejerk response to the issue which means that it gets ridiculed, such a stringent application of the law leads to those who are supposed to be protected by it being marginalised.

Now what you describe does not sound any worse than Evita and that got published legally, albeit in a different time. IN fact, because you are not being overtly sexual or pornographic and are, I assume, talking about a male character (cos, you know, despite equality, men who have promiscuous sex are still studs while women are sluts) then you may not have all that much of a problem. Legally, that is... after all, as things stand, talking about it or writing about it is not a crime. I could write about robbing a bank or murdering someone. Writers do that all the time. We could not have crime dramas or mystery shows or CSI if this were not the case.

Pornography laws may be a different matter... though I say pick up a Laurell K Hamilton or Charlaine Harris book (both aimed at teens and older, both published widely and bestsellers) and see what they write about in terms of sex. Its actually quite graphic in places but they get away with it because its in writing not visual imagry.

Your big problem may be scandal. Evita is one of the most notorious books ever written because it includes underage sex. If this is a major part of the book, something you may have to talk about on book tours and in interviews when doing publicity, then you are going to have face flak for it. If its a minor part of it then you may get away with it but it all depends on the mood of the time. The best people to talk to about this are publishers, especially any publisher you are thinking of approaching. What is their policy? What are their guidelines? Are they prepared to take on your book with its potential publicity pitfall? You may find that they get a lot worse sent to them anyway. Frankly, I can think of one book and one book series off the top of my head which end with two underage characters having sex. One is Hermetech by Storm Constantine (which I just finished reading) and the other is the Dark Materials series by Phillip Pullman. In both cases it was essential for the plot that these characters have sex and in the former it is blatantly described (plus cases of the 14 year old heroine drinking, smoking and flirting with older men) while in the latter (aimed more at children) it is merely heavily implied and happens 'off screen'.