This is a tough one and I think for me there would be several different factors involved.
Legal. If it's depicting an act that is illegal in *any* Western country regardless of where the story is set or the nationality of the author, then I would want a chan warning on it. (I exclude fundamentalist dictatorial states because they probebly wouldn't get net access to OTW anyway). In other words, even if the story is set in Japan where it's legal at 13, but may be read by someone in the USA where it's not legal until 18, then anything involving a person under 18 should carry a chan warning.
I would not label a sexual act between two teens of approximately the same age as chan, even if they're both underage, although it ought to perhaps carry a different kind of warning.
In a "Claudia" type case, or the chibi-type case that was mentioned previously, where a character appears to be a child but isn't, I would want to be able to give a warning something like "looks like chan but isn't"
If the characters are non-human then I would want to consider the normal aging process of the characters themselves. For instance in Middle Earth, elves are considered to reach the age of majority at 50, which is the equivalent of a human of 18. In the Wraeththu stories, a Har reaches adulthood at only 7 years of age, so a har of 8 is an adult. If the race involved is one for which the age of adulthood is not defined then I'd consider the way the character is portrayed. If one of them is portrayed as being childlike or like a teen, then I would want a chan warning on it. (If the characters are so non-human as to be more like animals then I think it should be bestiality labels being considered, not the chan ones).
There should also, with any warnings, be a way to define the difference between "explicit" and "implied". Thats where ambiguities happen - some peole warn perhaps un-necessarily where chan (or anything else) is implied but not explicit. And some readers when confronted with a fic implying chan, but wasn't warned for, get upset. So I think it would be a REALLY good idea to have a way of labelling the difference between implied chan and explicit chan.