If a character canonically is under 18, or looks under 18, then I won't touch them with a ten foot pole. That's one of the reasons why I don't use Rikku from Final Fantasy X (even though she looks as mature as the other adult characters, she is canonically 15 in FFX, and 17 in FFX-2), or any of the Life is Strange characters (even though they're all canonically 18+, due to their art style, they seem more adolescent to me, close to 15 years old - an ironic inversion of the Rikku situation).
Marie doesn't fall into this because, despite being petite, she still has the physique and head:body:extremity proportions of an adult woman. A friend I went to university with is 2 years my senior, and she looks pretty much look exactly like Marie, minus the fact she's brunette and wear glasses. Same soft round face, a stick of a body, and hilariously short. She also loves to rant on Facebook about being 28 and still getting carded whenever she goes to a bar.
So beyond that, I'm more or less open to anything that doesn't violate the "Insides should stay inside, outsides should stay outside" rule. The one exception to that being semen, which I think you can pretty safely argue is an inside that's meant to go inside someone else, and so it being outside isn't really too much a deviation (just means you're really bad at making it go inside someone else). But other things like urine and feces, those aren't sexy; neither are things like prolapsing and the like.
The content I will create is a bit more limited, just due to the fact that my works are funded by Patreon, and Patreon has more explicit rules. On the subject of, when I reached out to Patreon some months back for clarification as to what is and isn't allowed, the only thing they weren't ambiguous on was incest: they were extremely explicit when they said that incest is just straight-up banned in pornographic art.
Beast stuff is similarly hugely frowned upon by Patreon. The response I got suggested that if you can prove that the partner is sapient (capable of speech, reasoning, and consent) and is nothing more than animal-inspired, then it's probably okay. But, again, most of their response was ambiguous, and so unless it's something that's clearly not an animal, like a Krogan from Mass Effect, I prefer to err on the side of caution.
That isn't to say that I won't make things like that. But any such content won't be supported by Patreon (no wips, no teasers, no early access / 1080p), which means they take on a unique financial risk in creating. I do have some content planned in that vein (eventually), but that's all pretty far away. Ideally I would like to get in a more stable production position, with some additional people helping me work on things. Most of the projects involving these things are films, too, so that means that they're several years away - no earlier than 2025 likely, since I have to finish both Hiveship and Blue Star Episode 4 before I can even contemplate one of these films as being my next Long project.
Obviously this isn't a catch-all for what I will won't do, but I think it's a good general rule of thumb.