Side note: in a module I wrote and ran a few years ago I kinda did just this: a big ol' Blue Dragon that in her prime would have summarily slaughtered the party was, when they found her, falling apart due to what seemed like sheer old age. Blind, decrepit, slow, arthritic - she was a shadow of her former self physically. She still had her brains and guile, however, and still breathed lightning for a rather stupid amount of damage (save for half) thus was still threat enough to drag two PCs down with her.
Later they found her wretched condition was caused by a very magical crystal (which also happened to be what the party was there to find) she had eaten 50-odd ago that had slowly been killing her from the inside out ever since.