I've posted about this before but it continues to fascinate me. I'm also not talking about the more obviously fucked up topic of the sexualising of teenage girls in Japanese media, but what constitutes an "older" person over there.
I'm currently watching the Netflix adaptation of One Piece, and saw some Japanese discussion about the show's portrayal of Shanks and the actor playing him, who looks like this on the show:
There were comments saying that he "looks too much like a man past his prime" but also comments like "I like seeing attractive older men in media" and I'm just confused since he looks like a perfectly normal handsome actor man. They talked about him like Western social media talked about a 65-year-old Jeff Goldblum
I guess you turn into an ossan immediately after your 25th birthday
I think the thing with the young protagonists in Japanese pop culture is that they're supposed to be relatable to and reflect the age of the target audience. Most big Japanese franchises are aimed at 8 to 18-year-olds.
In that aspect it's not that much different with much of Western pop culture with superheroes, Star Wars and so on, but in the West kids have had no problem liking grown-ass adult characters like Superman or Indiana Jones or whatever and them being grown-ups is a big part of the appeal and power fantasy.
I don't think that fully applies to what I was talking about. I don't mean "young plucky hero takes on the world" as much as the much wider spectrum of near-total preoccupation with female children or childlike characters in isekai/harem hog feeding products. There isn't even really protagonist agency for those characters there: there's just trophies to "win" for the male ego-insert, young or otherwise.