The problem here is hentai content based of anime characters. Most of these characters are under 18 canonically and thus any erotic content wouldn't be allowed under current ruling even though they look like and act like adults.

Then there's the gray zone for characters that have progressed or regressed in age in some way, a good example is Ciel from the visual novel Tsukihime who technically is past her 20s, but her body stopped aging at 26 due to lore reasons. Under current ruling it would be hard to be certain whether it's allowed or not even though she both looks and act like an adult (although a weird one as Nasu can't write a proper R18 scene to save his damn life). Another issue is aged up variants of characters where their canonical age is way below 18 but the art depicts them as above it.

That said I do stand by enforcing an age limit for real people, but when it comes to fictional characters (and anime characters in particular) it makes more sense to go by appearance, this rule would include all these characters while still disallowing Loli/Shota which is the main concern for other instances (see burggit for example). This draws a clear line with less ambiguity than "they are a child" as not everyone may have the same opinion and this could lead to more things getting hit in the crossfire than is necessary, yes I know this leaves characters like Megumin in the gray, but it's not as big of a gray zone as "not a child"