Age is just a number and writing has little room for numbers.
True, but you do have Word Count, Chapter Count, Page Count, Planning, Time, Musical Inspiration. I mean that's a lot of numbers.
I think it'd be nice to play a character which can take the role of "Adult with little experience" than "Child who can do nothing." Children are used in fiction as a part of provoking our protective instincts, why do you think Charities appeal to you by talking about the poor helpless children? Children seem to take 1 of 2 things usually, "Deadweight, Annoying, Silly." or "VIP, Protect at all costs" and when I was a child, which is quite recently comparitively, I didn't like being treated like I was stupid or inferior due to my age. I'd try to debate with someone and they'd laugh me off. I think one scene where you can get the protective side and suprising "They're not useless" is if you had a Zombie setting. I wrote a fanfiction for a Browser game called Urban Dead once, and one of the characters was a little girl, and when the main character went in to a building to get something vital, he came out and found the girl fending off the zombies because she'd handled a crossbow before for sport. Children vary but People vary too.
You have got the issue of Pokémon and yes that does show to some extent the actual autonomy that children can do, but is dwarfed by the world being utopian.
When you have older characters I feel like they can be used to great effect to challenge preconceptions of the little old person and show that people don't become vegetables once they reach a certain age. Returning to the idea of zombie settings, it's because the art of survival brings a whole range of things to the table: Adrenaline, necessity, co-operation, trust, sadness, joy over that ember that you gather around. Instead of years as just a number, I tend to think of them of measures of how experienced someone is. Why shouldn't a old man know how to lock pick? Why do old people have to dreaming life away on a arm chair babbling on about "The War." Why do children get cast aside because they're just children? Yeah, children can be difficult to play, but people are immature all the time. You might argue that how dare we talk about having children having horrible lives in such violent conditions in things such as these roleplays? They should have a real childhood! But the truth is that these characters are things that can tell us things, in the world there are tons of bad thing happening all the time, whats to say that we can't make people wake up and see a new perspective? Childhood and Old Age are just as precious as adulhood.