@MushroomPersonally, I wouldn't worry about it. First off, "did it with" can mean a lot of different things (intercourse, oral, make out or just some random kid bragging about something he's never actually done). Plus, you were right, there is no description of the actual act, just a casual reference to it.
Now, if the kid keeps making that or similar claims throughout the story, then you'd be 'painting a picture of repeated underage sex', where it's more complicated.
Normally the 'off-screen' rule describes a random event in their pasts which describes what made them respond how they do (i.e. references to rape, parental abuse, going on the lamb or just being 'troubled'), whereas a continuing list establishes a 'well-established pattern or criminal activity. It's still covered by the 1st Amendmentβthough currently, that doesn't mean what it once did in a while varity of previously 'free' countries.
So if you just make a single reference, no prosecutor is likely to make a big campaign over it, after all, they're trying to promote anger about the outrageous actions. And currently, there's a bigger outrage over transgendered 'people'.
And of course, since no one ever scans stories offenses for keywords (other than cases like the one on SOL recently, where someone in a country where it's not even a crime, decided to threaten to expose sites supporters. Which again, was a completely bogus, empty threat).