<Snipped quote by Captain Jordan>
If it's a sexual roleplay? Don't join until you're 18. As said earlier, laws exist, so that's just a bad idea. If it needs you to be mature enough for the topic, then just be mature enough for the topic. The latter shouldn't have any kind of age even mentioned, because maturity has nothing to do with age.
That wasn't the question. The question was about a roleplay which requires their players to be age 18 or older. I'm not sure we should be encouraging players to disregard the rules set out by GMs. Doing so is disrespectful to the GM, the fellow players (who are following the rules) and to roleplay as a whole.
Disagree with a rule? Okay, no problem. Everyone's welcome to their opinion.
Request a GM changes a rule? Sure, if you think you've got a reasonable request.
Completely disregard the rule and encourage others to do so? No, no, and did I mention no?