@Dinsdale
I'm also thinking this is "19 but looks like 13" so age rule violations do not come into it.
It could really go either way depending on the story. Without reading it I'll suggest a couple of alternatives:
1. The character is consistently described as "looking 13" and either there are no specifics or all descriptions focus on the apparent youth. Everyone thinks she's pubescent unless they know her personally.
2. The character (or her family) worries about how young she looks, but actual descriptions are more complex. Note: I have an acquaintance who falls into this category; while it's true that she has very small breasts, she doesn't look young so much as she looks wrong. Sort of like someone with disproportionate dwarfism, although the girl I know is around or over the upper limit for that.
3. Descriptions of the character's personality could focus on how distracting it is to see an apparently young girl acting so maturely or they could simply express her personality, with only newly introduced characters commenting on the oddity.
4. Descriptions of sexual content could focus on elements that make her seem young: how small she is, how tight she is, her slim hips, etc.
5. Alternatively, sexual scenes could focus on emotional elements of sex or the actions and sensations of the act overall rather than only specific features. An analogy would be a sex story in which a character is overweight, but the author doesn't explicitly write the scene for size-fetishists.
I could go on, but I think you get my point. A story about a 19yo who looks like a 13yo could easily violate the 14yo rule, but doesn't need to. A lot would come down to the details.
Of course, I could also see a story like this being banned outright, regardless of whether it technically crossed the line or not. Although in this case the whole discussion is irrelevant since the story in question was published in 2007.