This attitude that some people have has always absolutely baffled me.
Parents in particular often do this one. They'll say things like video games or whatever are "childish' or some stupid crap like that..... but then THEY will go do things like, sit in front of the TV for 4 hours (which isnt even interactive! Agh! It's freaking mindless, by my count), watch football or some other sport (basically, watching someone ELSE play a game. A game which everyone and their grandmother completely freaks the heck out about), go to movies, drink alcohol (I'm not even going to bother explaining what's wrong with that one), go to concerts (well, my parents do this one at least), and stuff like that. They do PLENTY of things that are, essentially, pure forms of entertainment. But those are considered "adult" because...... er..... because.
But stuff like gaming or anime/cartoons or reading comics/manga..... yeah, SO FREAKING CHILDISH!! !
Which, to me, only proves that the person saying that knows nothing about the topic at hand.
As a heavy gamer myself, I can safely say that the hobby (and most similar hobbies) can be for ALL ages. You dont have to be a kid to get into these things, and indeed, there are PLENTY of things within these hobbies that are NOT appropriate for kids. And even among those that have no offensive content, there are plenty that are STILL not geared to a younger audience, due to being hyper-complicated or whatever. There's all sorts of reasons.
All it really is, is simple: It's a generation that did not grow up with these sorts of things, and frankly, doesnt UNDERSTAND them. There are exceptions, of course..... and as time goes on, I see more and more "older" gamers (or whatever) appear, but still. It's really mostly just a matter of people not understanding something, but thinking that they do.