Sometimes anime and manga as a whole are criticized for having too many stories take place in high school or involve high school students, and indeed there are a lot of titles, both good and bad, which fall into that category. While explanations range from “that’s how old a lot of readers are,” “there is a certain ideal to high school, the moment before you become an adult,” and even “so they can sexualize teenagers,” I have to wonder if it has anything to do with high school as a point of commonality among Japanese people.
Most young people in Japan go to high school (if someone can argue otherwise, please do), but once they go beyond high school their lives start to branch out more. Some go to college, some enter the work force, some go to technical training schools, and so on. This is even a plot point in some manga which make the transition out of high school such as Initial D. What this means is that, as a writer, if your aim is to have a position in life that the majority of your readers can directly relate to, then that period becomes harder to manage because not everyone will have that roughly similar experience post-high school.
Obviously this doesn’t mean that people cannot relate to characters outside of their own experience, or that people will reject heroes in unfamiliar settings, but that you end up losing that simple and easy connection. Such a loss can be overcome and frequently is, but high school perhaps remains that time people can look back to and say “I lived in that.” They might not have the magic powers or have gone to the rich school where everyone eats diamonds, but there is the thematic shorthand nevertheless.