I would say that if humans/humanoid beings are involved it would be both the mental age and physical appearance. Even if the person involved is much older than they appear, most people can't help but view them on some level as a child, simply because of their appearance... Interview with the Vampire comes to mind; you said something about "vampires in child bodies" and it made me think of Claudia. She looks like/is described as looking like a child throughout the whole story; you know that mentally she's much more mature than this, and thus is capable of doing things like killing in cold blood, but at the same time I think most people can't help but think "wow, but she's just a little girl." I imagine that sex wouldn't be much different. The fact that she looks like a child would probably be enough to squick people who don't like chan (or to trigger someone who is triggered by such things), and so fics with situations like this should have a warning for that reason alone.
The opposite situation (adult body/child mind) should too, but for a slightly different reason; specifically, it would be questionable that a person in that state would be able to give consent (or understand what they're consenting to.) For the sake of keeping things uncomplicated, I would suggest that if a human/human-appearing character has reached puberty and knows about sex and "the facts of life", then a chan warning would probably not be necessary.