I think if you're 17, days away from turning 18 - then you should talk to the GM. They might reserve a spot for you, insofar as hot accepting a full cadre of players just yet.  

I think though its the last bit of the question that gets to me. Which is essentially "If I pretend to be 18+, cam I just ignore the rules" The answer to what is simple. If you act 18+, can you just go buy cigs? can you go to a strip club? no. No amount of acting is gonna make that okay. Ever. Mature or not.