Thrillhouse- there is nothing wrong with saying "I've outgrown drinking." I'm not even 21 yet, and I feel like I have. The difference is that I realize it isn't my place to make anyone else feel immature because they still like to drink. I don't think it makes them immature, nor do I think it makes me more mature. It's just a difference in what is considered to be a good time.
I drink rarely... but it's still a good time to go out, and I don't get bothered by other people drinking around me. I'm glad they are having fun
I can have plenty of fun without alcohol.