This bothers me a little, because it's confusing and it pulls me out of the story a little. How can I fully relate to a character if I'm not even sure his age is real? Every single time I read about a character like this, I'm reminded about the Weasley twins trying to enter the Tournament.
Luckily it seems some authors decide to "age" their characters, so they appear much older than they really are. I don't think I've read books in which characters act like they're a lot more younger. It would be a little too awkward if that happened.
Don't get me wrong, if the "don't act like your age" thing has something to do with the plot, then I'm okay with it. Think spies. But if it's something that has nothing to do with the actual story then I'm confused. A five year old shouldn't talk like a ten year old, a teenager shouldn't be acting like someone in their mid-twenties and so on. And it's not about those moments or scenes where sometimes the brain decides to go offline for a few precious moments/hours. I've had those too and I think everyone has those moments. And I'm not talking about geniuses either. I'm talking about constantly not believing the characters have the age they supposedly have.
What actually confuses me the most is the why behind this. Is it a choice made by the authors to make their characters act like this (symbolism maybe?) or is it just an "oops"?