You won't have a problem finding players for a Mature(18+) game...not at all. Heck, EVERY game run by the Magpie team - of which I'm aware (I ran 4 for them last year) - is marked Mature and they were all filled.
As David Campbell notes above, the Mature rating is a guideline that is not enforced by Gen Con. If you are running with a publisher like Magpie, they will manage that for you i.e. if you are not comfortable running for players under 18, they will step in and defend your right not to run for those players. (HIGHLY recommend Magpie for their GM Support).
However, if you are running independently (as you appear to be suggesting) you will need to be comfortable addressing that issue with the individuals who show up who are under the age limit. Stand your ground. You have a right be comfortable as well. Just because Mom/Dad insists their 12 year old is "mature for their age" and "plays CoC all the time" and "I'm their parent, I give them permission" does not mean you have to accept them into the game. Just prepare yourself mentally for this to happen and hope it does not.
As a gamer who seeks out Mature games at Gen Con for *exactly* the same reason (because I want to play with and make friends with adults who I might want to share a beer with later not because I'm interested in sex, violence or other craziness), I would applaud your stance as a GM if I were in one of your games.