I feel like some creators are downright terrible at making characters match their ages visually.
Earlier this week I found out that Azula from
Avatar: The Last Airbender is 14 years old during the show. That's absolute insanity. She looks, sounds, and acts like she's at least college-age.
Fiction in general has a problem with making children/young people look, sound, and behave their age. It's hard to write for a 14-year-old, or a 6-year-old, when you're much older than that and your brain is more developed. Pay attention to what little kids do and say in movies/shows/games/books the next time you see one, and compare that to little kids you see in real life. The difference is noticeable, especially if you have younger siblings or something.