@So-chan
Not all kids are like that.
If the kid is: mature, self-aware, shy, or depressed, they don't act like that.
Plus, my version of tantrums growing up was going to my room and reading a book and ignoring everything. I can bet you there were kids that throw tantrums in a way not easily noticeable, or their way of "acting up" is a lot different.
If we were to talk realism, I feel like Naoto would be the equivalent of how orphans mature much younger. Neither of his parents act very mature at home and might end up forcing Naoto to take that role, which is consistent with the rest of the story. I know a few bookworms at the library I frequent that acted exactly like Naoto, albeit 3 or so years older than him.