From my experience with B8M8Network.com, I believe as long as the individual has stated that they are of legal age, they can join. But the existence of anything that appears to depict underage guys is reported, removed and the user banned/reported.

I have always maintained that access to sites should be managed by the person who owns the PC or the Parents/Guardians of kids.

All webmasters should take basic steps to prevent exposure of their site to kids, but there's only so much that can be done. The rest is up to parents/guardians/PC owners.