Maturity does not necessarily correlate with age. I've known enough mature younger people, and immature older people to substantially change any preconceptions I may have had that older people act more like adults (which is a very vague adjective).
Then again there are also many different concepts of maturity. For instance the kind of immaturity you don't want is the type that creates a huge ruckus in a fine dining establishment and gets you all booted out. But there is also immaturity that can make people laugh and help them to have a good time.
I don't quite see why it needs to be 21+ unless you're going to a bar or a venue which only allows those who are of drinking age.
If you don't mind my input, one very nice restaurant I've eaten at is the
E&O Trading Co. Sort of a very nice pan-Asian restaurant. I would suggest semi-formal wear. It would be silly to come in cosplay, especially if your cosplay includes very large and bulky props. Probably not best for a huge party, but if a lot of people want to go, splitting into two or more parties is perfectly okay too. It's best enjoyed as a meal eaten over mid-mature conversation. You don't need to be silent and a wise man, but you shouldn't act like a little kid either, and make loud raspberry noises, or farting with your armpit.
You'd have to bring about 20 bucks for your meal. If you're 21 or over, they serve drinks, and also have a bar. It has a VERY nice ambiance. I've only eaten there once, at my first Fanime in '04 so I haven't had recent experience with it, but what I remember was that it was an awesome restaurant, and great food.
Also good if you just want a quiet night out with a special someone.