Cats can experience cognitive dysfunction, just as we do and dogs do in old age. http://www.petplace.com/cats/cogniti...ion/page1.aspx
Our Ginger is 18.5 and is quite deaf, though she can hear. if you look at the link, Ginger has shown the last two and decreased playfullness for about two years now. Cats depend on their hearing a lot for play so I don't know if that is due to hearing loss or C.D. for our girl.
Ginger always was a very vocal cat but now it's terrible. Her tone is loud and insistent and demanding and painful and much more frequent and long lasting. Partly I suppose because she needs to be loud to hear herself. Ginger will walk past us in the living room, say we are sitting reading the paper. She'll jump up on the chesterfield, yes she still can, but then she will face the back and HOWWWWWWWLLLLLLLLLLLLLLLLLL. We will yell at her, GINGER, we have to yell, and she'll look around in surprise as if to say, "there you are." And she will have walked right by us 30 seconds ago.
She definitely has the nocturnal separation anxiety. More HOWWLLLLLING.
On the other hand she is not as easily irritated by the other two cats but I think it is because she doesn't hear them coming in time to be nasty to them.
Other than some minor kidney failure, some arthritis for which I give her Sub-Q cartrophen Vet, which helps enough, she is fine. No bathroom problems and she can still get up and down on her cat tree. She does not groom as much as before but that must be partly to the arthritis. She still keeps her white tummy sparkling.
For your Tabitha, is it at all possible to put the litter box closer so she can find it more easily? It's not easy watching them age, is it?