I know I'm new, but this particular topic hits on something I've been doing myself. I've been an admin on an Ultima Online community for 10 years, specifically with a "mature players only" policy. There are two problems we are constantly coping with.
1) Immature players ALWAYS sneak in. They lie about their age, they act different until they get in, all kinds of ways they get in. Because they WANT to see all the "mature" stuff going on, they don't think it's fair to be excluded, or they want to disrupt the "mature" play because it's "fun".
2) It's a TON of work to maintain. Everyone has a slightly different idea of what it means to be "mature", be it personal definition, cultural bias, what have you. So there is always friction about someone being "immature", someone crossing someone's personal line, etc. It's exhausting.
In spite of those issues, it is certainly possible to run and maintain a "mature" community. But from my personal experience, it's an all or nothing deal. You either run the whole show that way, or you run the whole show a different way. It's easier across the board, it's less exhausting, and you have far fewer loopholes for people to exploit behavior wise.
My personal way of dealing with it, I just behave according to my definition of "mature" and go from there. You still have the occasional "immature" distraction to work through, but a lot less of them than if you publicly set up a known "mature" set up.