It's this exact question that made us seriously consider just banning hentai altogether. That would have been the safer move, for sure, but we were worried it would cut off too many people from a safe, accessible place to do porn things.
So context matters, right? Like we're going to have to trust that the mods of hentai communities are good at hentai, so they know better than a clueless layperson what's actually supposed to be underage and what isn't. For me, looking at hentai as a guy that watched Ninja Scroll a bunch when he was 15 and then never really went back to that well, there's stuff that's obviously forbidden, stuff that obviously isn't forbidden, and stuff that falls in the middle where I have to be like "Well, I don't know who that is, if their show is aimed at kids, if they're supposed to be a kid, if within the context of hentai that outfit and those visual cues indicate underage or not, so I guess I have to trust someone that is more of an expert". Because if you force me to make that choice, I'm going to err on the side of caution and nuke everything that's more questionable than not, which is probably like 75% of all hentai.
But with a knowledgeable mod team, if they tell me that something isn't underage and isn't meant to be underage, I'm likely to trust their judgement even if I myself as an ignorant not-a-fan-of-big-eyes-small-mouth-tentacle-sex-cartoons would have made a different call. Unless they start making blatantly bad calls, in which case then we consider taking more drastic action, we are going to defer to the experts.
There are also issues of cultural imperalism and racial bias involved, right? Like studies show that Americans of different races consistently rate depictions of black people as older than they really are and asian people as younger than they really are, and on top of that trying to overlay Western artistic values on top of Japanese cultural constructions can be kind of fraught (and I used to live in Japan, where at 15 I was 6' and 180 lbs. I was much larger than like 99% of the the native population and they all thought I was a grown-ass man, so I know how that kind of thing works albeit in reverse).