Not be be a dick here, but how mature you see yourself to be has no bearing on whether or not you'll be allowed to play. Every 12-15 year old that comes here makes the same claim, and more often than not, they're eventually throwing tantrums when they're told that they can't play or get a gun and get banned. Not saying you'll do that in any way though. Just saying that this is often the case. Also remember, what you , at 15, determine to be "mature" and what adults consider mature can be 2 very different things. Become a productive member of the community here on ASC. Once we get to know you, we'll determine that for ourselves.
As for immature jokes, you'll find that most airsoft players are a bit offside with off-colour senses of humour. Take a look in the "You Laugh You Lose" thread in the off-topic area, and you'll be able to see that for yourself. So in this community, that can actually be a plus...