In the US, depending on local attitudes to some degree, it is quite possible for a juvenile to be "tried as an adult." Up to about age 13 or 14 that would be unlikely except for exceedingly criminal activity, but given attitudes about juvenile sex it could happen.

It is NOT really uncommon for males at 16 and a day and older to be "tried as adult" for actions where the judge believes he "acted as an adult" (did an adult thing) and should have known better. That raises the odds tremendously.

The female in a situation such as this, appearing before some judges, would be deemed a juvenile until she's about age 30. At 16 or older, a boy could easily be tagged as a "sex offender" for the rest of his life. I've never heard of a juvenile female being similarly charged, much less convicted, but would suppose it could happen.

As to what you should do, it's unlikely that anything you say overtly to either teen is going to have any effect, other than possibly to encourage them to "do their thing" under even less favorable conditions or in more hazardous places. Telling anyone else greatly increases the likelihood of overblown and drastic (likely legal) actions that could be quite damaging to one or both of the kids.

Several decades ago (times and attitudes were a little different) an uncle who had "covert knowledge" of something similar just handed a condom to the boy, said "Here" and walked away. Nothing more needed to be said - and there really isn't anything more you can say, that is likely to have any constructive effect. (It helps to not smile with this delivery, though - I guess.)

It's an entirely different matter if it's one of your own kids. In this case they're not.

John