I just ran into a similar problem with one of our clients. She wants me to set up a message board that will have content that's considered for "mature audiences only", but not the adult porn type. She wanted me to come up with a login system that would allow only adults in. I told her that I don't think there is one that can guarantee the underage will be blocked out, even if we asked for something like a credit card number. Knowing my clients way of thinking, I think she would like your idea of some kind of questionnaire though.