No, my cousin had something like this girl. When I was a kid I just figured he was mentally retarded or something, but then my parents said "no, he has some disease where he doesn't age". He might have also been slightly retarded too but I don't know. He died a few years ago and I think he was 12 or something, I'm not sure. As I said he was alright. It must have been a strain on his parents, I felt bad for them after I knew.
I know what you meant, I was just pointing out that there are others who seem to have the exact opposite condition. Their mentality developed normaly, they acted like an 8 and 12 year old would, but their bodies were aging rapidly.