Claudia's definitely who I had in mind, but there are several other fictional vampires in children's bodies.
human/human-appearing character has reached puberty and knows about sex and "the facts of life"
There's two issues with that (and I bring these up just as side thoughts; I'm interested in everyone's judgments even if I'd decide differently):
1) "has reached puberty" is nothing like "has reached legal age of consent;" there are sexually mature & active human 13-year-olds, which many people would consider chan, and
2) some who have reached puberty or even adulthood have no idea how sex works.
This includes "raised by aliens" scenarios, mentally/emotionally damaged characters, cultures/races where sex is supposed to be a total mystery until it happens, and
There's another issue: fictional characters associated with children, whether or not they are children themselves. Especially characters who don't express any sexuality in canon: Bugs Bunny, Snap Crackle & Pop, Barney, etc--"icons of childhood" that are commonly associated with innocence, but aren't exactly "young."
And does that mean non-human(oids) are exempt from the "chan" label? Dragonfic doesn't have chan?
I suspect that the "chan" squick is more a case of "fic about sexualizing innocence" than age-based, and as such, is likely to be very hard to find objective labeling standards for. And that's aside from the disagreements about the relevant ages for humans.