I figure having it be a congenital disorder would help explain why it can't be fixed with magic, so that's what I've been focusing on. The rest of the details are as follows:
- The noble girl is known to be ill by the time she is four years old, though earlier signs are perfectly fine.
- She dies, either directly because of her condition or from complications resulting from it, at eighteen years old.
- Ideally, the disorder should be one that restricts her ability to run, play, and roughhouse with other children.
- Symptoms that come and go in fits would be excellent, but are not required.
I've done research on Wikipedia and by Googling stuff like "congenital lung diseases" or "congenital heart disease". I've also searched through WebMD. Most of the information I've seen, though, assumes access to modern medical technology and doesn't say what the prognosis is without it.
EDIT: Wow, thank you all for getting back to me so quickly! I think cystic fibrosis is a great idea, and yep I do think I can say her parents were able to use magic on the symptoms to stretch her life out a bit.