The same thing that prevents them from literally any check at any level you’re doing from just saying “yea I’m 18”.
There’s literally nothing you can do to prove someone is a child that wouldn’t require an extraordinary amount of effort and be required to be utilized on every single person. Even that is simplistic to get around unless you utilize such a level of intrusiveness that no one is going to bother doing it for your webforum anyway.
Your only real options are be incredibly intrusive to the entire community (eg. we randomly check all private messages and block any references to outside communication) or trust people won’t lie and follow your environments in the ideal way