I'm concerned about how enforcable this would be. There are tens of thousands of fictional characters and 90% of underaged anime characters have the same look as any adult character.
I don't know anime all too well, so I typically go by a rule of removing posts if their age looks questionable, and banning users if it's full on creepo stuff. Should I be expected to do more than that? Looking up characters for every single post is not going to be feasible, especially on the larger communities and on communities with small mod teams.