Hi Kenzon.
Always nice to see this sort of discussion, and for the record I like meeting another Avatar fan. I've been a fan of last air bender (or legend of Ang as it was markited in Britain). I just watched the first series of Kora a couple of weeks ago (was waiting to buy it on dvd), and thought it was extremely awsome! Aman is just plane scary, and actually i can't wait to get the rest on dvd.
Either way on the entertainment front, the question is more one of good story telling than anything else. Back in the 80's and early 90's, stuff that was aimed at kids also tended to have enough by way of character, symbolism, interesting world and dark storytelling to appeal to older teenagers and indeed adults. Something like the original Dungeons and dragons series or even original transformers was always written very well, and though it didn't go into ultra gore or adult themes it was expected that young adults as well as kids would be interested. I got the original D&d series on dvd a couple of years ago and it's really quite astounding just how well the dialogue, the world and the plot work. This is one reason wy for example very adult films like Robocop got tie in series and cartoons, heck Robocop 3 was an attempt to make a pg/13 rated robocop, (admitedly rather a failure but still it happened).
Unfortunately however, things started to change, mostly I assume because business executives just tried to churn out specifically designed things created by focus groups for each age group. Stuff like MEga man battle network which really didn't have much by way of character or plot and a very sanitized idea to violence.
This was one reason Harry potter was such a shock to many people, sinse Hp was a book series with child protagonists very much aimed at anyone who liked a good story, heck my dad who is in his sixties knicked all my hp audio books and thought they were great!
It's a question of where something is just plane well written, rather than it simply being written as to appeal to a particular set of people with a very standard set of symbols and methods, indeed this is why I myself tend to rather dislike superheroes and the recent trend of superhero films, sinse the characters, the situations, the ideas behind them are often very predictable and contrived and don't really have anything particularly unique about them in characters, or world, they're all a sense of style over substance.
So, in the sense of watching or reading things and being able to view them independent of intended audience or demographic, that isn't childish at all! quite the opposite in fact. Indeed, I would cryticize the kids in your class who laughed at the chap who liked a pokemon film for being too childish to try and understand what might be good about the Pokemon characteriation and just expect people to all like the same thing. That! is childish, simply thinking inside the box, and kids aren't the only ones guilty of it.
On the more general behaviour front, well it depends on what you could call emotional maturity, which has very little to do with physical age. I have met twelve year olds who I can talk to as adults, and I have met 40 year old idiots who think they themselves are the center of the universe and expect everyone to be the same as them.
For example, once at the bar in colidge I was ordering my drink and a chap (who must have been in his mid 20's), said to me "why don't you have a pint?" when I explained that I had tried beers but wasn't a fan, he said "but your a bloke you should! drink beer, if you had 20 pints then! you'd like it"
Now, anyone who thinks they should down 20 pints of beer just because they have a y chromosome and that anyone who doesn't do the same is in some way wrong, that! is childish.
With our dreaming and singing, Ceaseless and sorrowless we! The glory about us clinging Of the glorious futures we see,
Our souls with high music ringing; O men! It must ever be
That we dwell in our dreaming and singing, A little apart from ye. (Arthur O'Shaughnessy 1873.)