Animemes

I think the idea is that the majority of the consumers and viewers of anime and manga are teenagers. To pull in that audience, the characters would have to be relatively the same age for the audience to relate to those characters.

This could also be said of American shows tbh.

Teenagers have emotional growth that up the tension/perceived stakes

Culturally high school is like this wonderful time of freedom before working age begins.

A lot of these high school shows were watched by me as a high school kid. And living my shit life, they taught me emotional and social things I wasn't gonna learn otherwise. And I finished shows back then knowing even at 18 or 19 they would be obsolete and childish in my life. I was wrong, but I did grow out of anime by about age 22.

But as an adult now? Please make some adult shows. Some of the most intellectually challenging and creative works are centered around 16 year olds. UGH.

I've heard that a pretty big component of the sexualization of underage characters in anime is actually pretty similar to why casinos and their machines are so... slimy, as it were.

I'm not going to claim to be some expert on Japan or the anime industry, so take this with some skepticism, but what I heard goes roughly like this:

Anime is a risky endeavor because it's very costly to produce. Most anime will be costlier to produce than live action or even other forms of animation, and on top of this, the average consumer will not spend very much on merchandise, or manga, or blu-rays, et cetera, and will not stay up 'till the wee hours watching commercials just to catch a show live. Therefore, in an ever-saturated anime market, new anime increasingly have to walk a tightrope between not being patently offensive to 99% of their viewers, and being appealing to the 1% of "whales", to use the gambling term. The anime wouldn't be financially viable without both of these demographics.

And the types of people who would spend tens of myriads of yen on figurines and posters, evidently, are disproporionately likely to enjoy sexualized cartoon high school girls. Meanwhile most normal viewers evidently have a threshold of how much underage sexualization they can roll their eyes at or ignore, before it becomes too disgusting to keep watching. This especially applies if a large portion of the viewers are in the same age range as the characters, and so might themselves find the sexualization to be relatable; and this double especially applies in a very hierarchical society with very visible issues with misogyny and ageism in general, where people might generally be less critical of those things. And even within a production, even if the vast majority of an anime's staff are normal people, there could always be some small portion of the staff who are so-called "lolicons" who would put that type of shit into a production unquestioned.

Regarding why everyone is a high schooler, regardless of sexualization— The other comments I think have part of the story. Another part of it is just how high school is a universally relatable experience, since people's lives start to diverge after high school; and how the high school setting has just been proven to be a successful formula, and so they'll keep doing that and the "hit by a truck and reincarnated into a generic high fantasy world" stuff until consumer trends change — and the consumer trends will only change when someone actually creates an alternative, and God knows when that will happen. There are plenty of great anime where the main characters aren't high schoolers, but those are riskier to make.

One last piece to the puzzle concerns people like Greta Thunberg and Malala Yousafzai. Which is to say, that people just eat up the idea of minors saving the world in general, not only in fiction but in real life as well. Minors themselves like seeing people their age celebrated and empowered, and adults also like the idea of just a few kids fixing everything, instead of all adults collectively working towards a better and safer world. And adults also like looking back on their teen years or childhood as a time of freedom, even though in truth people are certainly no more free at that age than as adults. Realizing that fact for many comes with a feeling of responsibility to use adult freedom for good, and who likes having even more burdens and responsibilities? And realizing that fact also comes with the burden of actually making children and youth free, and who the heck wants to do that, right?

I dunno. This is a media analysis 60% based on a half-misremembered video essay, so this is probably a worthless contribution, especially when this thread is already like five days old.