Yeah, it's phpbb, but there's nothing stopping them from lying about their age...
I'll talk to the main admin and the guy who runs the site.
If someone lies about their age, you haven't committed a crime. Its kind of a slippery slope. You can run a forum fine, and allow people under 13 to join, without consent from their parents, as long as you don't know they are under 13. But if they slip up somewhere and break their cover, you either need to get a form from their parents or delete their account.
Also, you can't encourage people to lie about their age.
Yeah, its kind of dumb. There isn't really a real way to enforce it, unless someone actually gives you their age.