There are some types of dwarfism that can affect facial features and teeth. And there's the achondroplasia type of dwarfism that affects the shape of the upper head, but the lower face seems to 'age' in the average way.
From N's head and face I can see that it's the body that's affected, not facial features.
But not everyone is aware of these things. I've been grateful for the photos that were posted of the family of the actor who was in the Harry Potter movies that showed his actress daughter in year-by-year photos, and also the photos of The Little Couple family, as that's not something I've watched.
To be fair there are one or two rare conditions where a 14-yr old still looks like a toddler, even in their face. And more people are aware of the obverse syndrome where aging happens too fast and a ten-year old can have the appearance and physical problems of the average 70 or 80-year old person. So if the Barnett's assert that N has a condition that makes it hard to tell her age, then most people would simply accept that as they know these other conditions exist and they're basically being told that N has something similar...so why would they question that? I don't think it's that those people aren't 'reasonable' people, I think it's more that we're lucky here to have people posting links and photos and information so that we don't all have to go hunting for an hour or two, looking for conditions for names that are hard to spell and pronounce etc.