I for one don't understand how "Mature" can possibly be confusing. I mean, honestly? Hello! Mature content! You see it in warnings before some television shows and video games or movies. How does one confuse that and with what?! If you really must rename them though, why not go with something simple?
[A] Club (18+) and [T] Club
Adult Forum and Teen Forum
Explicit [A] and [T]
For something... creative:
Willy Wanker's Wonka's Chocolate Factory
The Secret Garden
Forbidden Fruit Forum
Lascivious Liaison