D'awww, that is pretty up there in age. Honestly one of the oldest I've heard of and she sounds like such a sweet corn Cob too. ^^ It's wonderful that she was able to educate so many people like that and I'm sure that she's given you many great years too.
Pets like that really are part of the family, so I'm very sorry to hear that she isn't doing well.
Without knowing for sure though, I wouldn't give up on her quite yet.
Have you offered her different meals or feeding methods? Or perhaps try raising the temps a tad if she's only hanging around the warm side... I really can't say though - If she's losing weight, I'd take her to the vet and see if they can find something wrong. Really the best route for answers.
Best of luck to you both. I have my fingers crossed for Cob~