Thinking about what Chip has said, made me wonder too what door this would open to obscentity charges... I mean if we adopt something where specific acts are for 21+ and other 18+ what is there to prevent a prosecutor from using it against anyone they bring up on obscenity charges?

Again I'd use the format that television uses. Movies and games really aren't applicable as a model simply because there is someone there to prevent their access to a minor (in the case of movies) and sale to a minor (in the case of games or cds) while television there isn't. It is aired and who watches what is beyond therir control, just as it is for an adult webmaster.

So I'd say using a similiar system where it is either PG, G, Mature or Restricted or XXX should be sufficient of a system.