It's easier to market to children and pedophiles than people with critical thinking skills.

Because anime is from Japan, and in Japan, high school students are not considered children. After middle school, they apply to go to high school. It's optional. Otherwise they start working. It's just like college in the west, but it happens sooner.

There is anime with older characters, though. My favourite is Boku Dake ga Inai Machi (lit.: "The Town Without Me" but released outside Japan as "Erased") which mostly takes place in an elementary school and with ten-year-olds, but the main guy is 28. He's been pulled 18 years back in time to solve the murder of a classmate which will somehow (he has to figure out how) prevent his mother's murder.

Bungo Stray Dogs has characters with mixed ages. Most of them are considered adults in the west. One looks really young. One is a girl in middle school. It's basically Boku no Hero Academia but for a slightly older crowd. Super powers are more taboo, so they take on the identities of famous authors, and their powers are named after their books. The main guy is named after Osamu Dazai and his power is called No Longer Human... and that got me to read the book. Wild ride. The real Dazai committed suicide similarly to the character in his book. The Bungo Stray Dogs Dazai is always trying to end his own life, but in the most painless way possible, and it's played as a joke. Just fair warning if you're not the kind of person who finds that funny.

SPYxFAMILY and its spinoff Buddy Daddies are both about adults, but in both cases, it's a pair of killers raising a little girl. So, still child characters, but the fandom only sexualises the adults. Because the girls are four and five. They're incredibly adorable, but it never gets weird. (Okay, in SPYxFAMILY, the main kid's best friend, who is six, develops a crush on the main girl's dad, and plots to make him fall in love with her, so she can be her best friend's stepmom. It's completely innocent though, played for laughs, and it ends well.