Overused high school/young person trope

I don’t watch that much anime anymore because most of the time it’s in a school settings and they make these kids/teenagers so dumb (yet so Mary sue) . Reading mangas gave you more variety of ages but then I realized that some MCs are unnecessary young. Like wdym they’re in a mafia, gangs, murderers, have a jobs, fight monsters, make business, and go to places an underage person shouldn’t? Couldn’t they just be at least in their 20’s or legal age? Wdym it’s for the readers their age to relate to? Ya think they could relate being in a fantasy world or being on a dangerous mission?? I didn’t know it was a thing up until I saw a trash taste clip talking about the unnecessary school setting. It’s ok to use this if it really is necessary for the characters but don’t use it if the story doesn’t even revolve around the school but rather outside sheesh, that’s just lazy introduction(most of the time). Anyways, any takes about it?