I'm part of a generation that grew up with the idea that certain kinds of "kid stuff" are things you do want to preserve past your childhood. The ability to play video games together, for instance, or even play video games alone. The ability to enjoy colorful flights of fancy of varying degrees of artistic competence and demographic intent. The ability to tell stupid jokes and laugh at them, too.
I firmly believe it is our right to enjoy this stuff as long as we can balance it against everything else in our lives. And yet despite holding down a succession of good-paying jobs, despite never once missing a mortgage or car payment in my life, I still get this annoying vibe from certain other adults that at some point I really ought to quit twiddling around with all this kid stuff I like and go pick up golf clubs like every other respectable adult. Yeahno.
What's weird is how things that harm absolutely no one the vast majority of the time, not even the person doing them, are singled out as signs of "immaturity." Not things like groping women in elevators and having a good laugh about it with your other (male) co-workers. Or engaging in socially sanctioned but nevertheless enormously destructive drug addictions. Or ripping people off and calling that "wealth creation" or "value extraction". Or savoring and enacting thuggish behaviors against people with the wrong color skin or funny last names.
None of that stuff is ever trotted out as proof of "immaturity". Heck, most of that is considered rite-of-passage stuff for our current concept of adulthood. Immaturity is always easy, harmless scapegoats like your anime figurine collection or your comic books.
I don't think real maturity or responsibility have anything to do with such talk, and never have. It's all about control and conformity. Comics and video games are low-hanging fruit that anyone can use for cheap shots about maturity. But attacking real arrested development is a social project. Having a generation or two of people who are that much less interested in being sold on b.s. versions of "adulthood", "manhood", or "womanhood" is a good start.