I think it would be great to have an 18+ forum to do stuff like trading the actual cards, etc.
That said, I don't see how they could verify it. The internet is so anonymous. Even if they required scans of some form of ID to join, a kid could just scan their parent's ID.
I've always got some extra VMK cards whenever I go to Disney for possible trades. I don't know what good that'd do since we can't arrange meetings. But I'm always hopeful I'll stumble upon someone else as crazy as I am.