who is (or apparently is) a child under 16 years

Again, how do you determine that? Sometimes it's obvious, sure, but sometimes it's not. Who decides the age of a fictional character?

One of the biggest and oldest furry art sites recently went through quite a kerfluffle recently because they tried to strictly implement such a ban ... resulting in mass outrage among users as moderators went around arbitrarily and rather capriciously deciding which characters did and didn't look like they were under 18. Lots of artists had their -- purely well-intentioned -- art deleted and were outraged at being accused of producing "child porn" when they had never intended to. Any characters with small breasts or short height (even if they were otherwise clearly adult) were likely to be targeted, drawing further criticism from body positivity advocates, who pointed out that moderators were de-facto identifying certain body types (which some real people have) as 'not adult' and 'not a real woman'. Artists who did fan art of the Pokemon franchise were particularly hard hit, because at least some of the moderators seemed to be of the opinion that pretty much all pokemon were always underage, and therefore always "child porn". And artists with a more 'cartoony' style tended to get hit a lot harder as well, since moderators appeared to believe that simplified characters with less detail meant they were younger. And then there was the diaper fetish crowd, who lost all their stuff, because moderators decided that any depiction of any character (regardless of any other age indicators) was definitely a child if they were in a diaper.

Thankfully, that ended, and decisively so. The site was reorganized, the policy was heavily rewritten (with user input this time), and the worst offenders among the site staff were all fired. Sexual depictions of minors is still against the rules there, but they instituted much clearer, more specific, and more objective guidelines for determining whether a character is or isn't underage.

And that's just an online art site, where the worst thing that could happen is that your art gets deleted and you get banned! I can't imagine risking prison time based on what some prude with a chip on their shoulder decides about the apparent age of a fictional character.