That's a nice idea, but think about it. This is the internet, no one will use there real age, well I mean to get into a age board like that people will want to see what's in there that arn't of age and all they need to do is just edit there profile so there so called "over 18" in there profile. I guess someone could make it, but it really wouldn't do any good unless you can trust everyone would tell the truth which I could never see people doing online ;_;.
