Dungeons & Dragons isn't immature. While many players are teens, there's a substantial minority of adult players--including whole families, five-year-old kids and all. (Naturally you have to get rid of some of the more disturbing parts for a five year old... no killing off one of the party's friends, reanimating, and sending the zombie to take them a message from the bad guys, for example... Hehe. Yeah, I play D&D and I am a certified Evil DM!) I absolutely loathe the money-grubbing way they've put out a new edition nobody needed... No more money to them; I'm playing with my old books.
Um. Let's see. I do have some mostly-teen hobbies. Lately I've been playing The Sims 2, mostly player-devised challenge games that all amount to "How close can your sims get to death and still survive", or even "and still accomplish things that are difficult to do in regular play". TS2 without challenges is like playing house, and you kind of get tired of that after a while. Well, I do.
I also loved Disney movies until my early twenties, when they started becoming predictable.
I read lots of fantasy and sci-fi. Apparently these are not considered mature, either.
Actually, I don't really have a lot of stereotypically adult interests; they don't seem very interesting to me. The only stuff I like that's 'mature' is unusual for adults, too: Crochet, physics, feral cat management, psychology...