There are many factors which play into this sort of issue, which undoubtedly makes this a difficult discussion.
But I think in general, (fictional) kids actually acting like kids is natural yet potentially annoying to watch. It’s expected for them to not think things through or to make dumb choices or to be emotionally unstable, after all. Still, it doesn’t make watching easier (or maybe it does if you like trainwrecks and watching things go wrong. Who am I to judge?).
Be that as it may, there’s definitely some sort of fine line that has to be toed in regards to little anime boys or girls. They somehow have to be cute enough to make up for their carelessness, their noisiness, and their ability to possibly dominate a scene (or even an entire episode) due to their inherently selfish nature. Otherwise, viewers will not hesitate to criticize the child character or the series itself.
Therefore it’s a bit jarring when a young character acts older than their own age. Somehow it’s both immersive and not immersive at the same time — either outcome heavily depends on the viewer in question, however.
As some viewers get older and older, they probably begin to forget what it was like to be young, which leads to them complaining that they cannot relate to the hyperactive child character (of course, there are some more melancholic younger characters, too, but I think in general most non-wrinkly anime characters are by default cheery). Whether or not one can relate to a character is in itself a bit of a shaky argument that happens to be entirely subjective, I must add, but it’s one some viewers embrace nonetheless.
The very age difference between character and viewers can create some dissonance, really. But when some high school kid is acting mature for his or her age, older viewers might begin to like and to take notice of said character. To them, this character is something tangible, even if the youth starts waxing and waning poetry and philosophy like he or she browses Wikipedia as his or her own hobby, because the character is considerate much like how older viewers have to be more mindful of more things compared to back when they were younger.
Of course, all of this is very subjective. I’ve known some mature high school kids. This one classmate of mine had a fully padded out resume with tons of job experience by the time she was a senior since she had worked such a large amount of jobs.
Essentially, what I’m trying to get at is that the perceived average age of a young anime character will obviously vary between person and person. Whether or not the character will be considered either natural yet somewhat annoying or “relatable” and somewhat unrealistic is heavily reliant on a viewer’s personal perception. What one’s snot-nosed brat may be another’s pensive young adult.
The reasons why a young character will also vary. I won’t really elaborate here since that’s not entirely important for this post, I feel.
It’s a little less confusing when it comes to older characters. The funniest instance I can think of is when a particular character, who is much older than how she or he looks, starts acting like their physical appearance. Vampires (particular vampire girls) are particularly susceptible to this, and I always enjoy seeing their impressions of the young as well as the moment their facade drops whether it be due to intention or accident.
There’s no black and white answer here. Often times a lot of things are never quite that simple. I just felt like writing something I have had on my mind before going to bed. Hopefully it made some sort of sense.
Hopefully I can also do some more writing and reading when I wake up in a few hours, too. I’ve got a lot of catching up to do.
Thank you for reading.