
What's she like otherwise? Her normal self or any other changes, like keeping to herself more than usual? What about the coat? Her temperature? Colour of membranes -paler than usual or normal? Eye colour, any change in that recently? There's several things it could be that aren't be easy to test for, or it could simply be old age. I have a couple of oldies, one of 13 and one of 15, and they eat less and certainly have got a bit skinny, sleep a lot more as well, but they DO eat and they're not just skin and bones. I don't think much of the runt theory, at this age it won't make any difference any more.
Have you tried boiled fish (coley or cod), sliced turkey, ham, chicken, smelly food like sardines? Feeding kitten food instead of adult food in cat food? (So that she can eat less but gain more nutrients as kitten food is concentrated.) I assume the mooth has been checked good and proper, not just the teeth but the gums far back as well?
Sorry for lots of questions!