I don't think there ever comes a point at which someone stops being the way they were and suddenly becomes someone else in taste and behaviour. I was waiting for some kind of clear-cut line when I turned 18, and there wasn't one.
And I don't think anyone really 'grows up', in the sense they become someone other than who they were as a child, I just think we shoulder more responsibility and we get tired and bored of the things we once enjoyed. But if we lived forever, hypothetically, we would grow out of everything, and probably back into them again. We lose interest in things, or we get all the enjoyment possible out of them and move on. That's a natural process I suppose.
I feel the same person I always was. But I was always a more mature kid even when I was in single figures. And I know adults that act as daft and irresponsible as people in single figures. I wouldn't say 'childish behaviour' comes from something like a taste in music or in entertainment. I'd say it comes from a general lack of understanding about the world and the way things are, or about what proper 'decorum' is. There are young people who are very astute when it comes to this, and there are older people who are pretty clueless about it still, and so end up acting 'childishly' (for their age).
Walt Disney makes movies aimed mainly for children, and children love cartoons, but without an adult love for cartoons, these things would never be made. Was Walt Disney himself childish for liking drawing cartoons and founding a company that was made them for most of a century? Why animation is considered totally childish I don't know. I don't find it something only children should watch. Although obv. it makes sense not to show young children horror films and such. When you're young you are better off introduced to the world slowly and gaining understanding bit by bit, not directly through 'an adult's eyes', which is what adult films are. The other way though, from adult-to-child's view has no harm in it, it might seem very simplistic to an adult but then, some of us like to see the world that way sometimes. It can get clogged with crap as an adult, that sometimes you long to remember what it was like when your worst worry was how far away the school summer holidays were.
One thing that bothered me lately was an article I got sent about how people who are older than 30 playing video games 'is creepy'. It made me laugh. So the expectations of what you should do and say and wear at a certain age are still there, although I expect my generation and all of yours are going to still be interested in the Internet and video games long after our 30s 40s and 50s. Maybe it's just this annoying lingering idea that 'games are for kids' and the 'Internet is for nerds'. It'll be gone soon. People are more open to new ideas these days, and age, gender and class doesn't seem to be such a factor in what we do. Thank God.
I mean, I'm now in my 30s. What is a person in their 30s supposed to do all day? I have no idea and I don't care. I just do what I do. I know there are people who are thrown off by my appearance, they think I'm a 20 something student and I'm happy enough with that label, because at that age you aren't expected to do much except be poor, get drunk, not worry, and have fun.
I expect I would change over time, start worrying about other things and wanting to do other things, but now I think I know myself well enough to see that this is how I'll be for good. Grow up? I don't think so.