Cedar, diabetes is a possiblity at her age. Another illness to check for is overactive thyroid (hyPERthyroidism), which isn't that uncommon in elderly cats.
OK, Squeaky, Thomas' supervisor, was born with micropthalmia in her left eye. This is a birth defect in which the eye doesn't form normally.
Squeaky's case, the eye was basically a blob of tissue made up of random, partially formed eye parts. In additon to being totally blind, and not very pleasant to look at, the eye was constantly infected and causing her a lot of pain.
Her vet and I, after fighting with the infections and trying unsuccessfully to control Squeaky's pain, decided to have the eye removed.
From Squeaky's point of view, since she never had vision in that eye, she went from her eye hurting all the time, to her eye hurting a LOT for two weeks (she had srong medications), then to finally not being in pain at all.
It was after the surgery that Squeaky's socialization was able to progress and she turned into the little lover she is today.
The sad thing is that Squeaky was approximately 4 years old when i adopted her, and had suffered since infancy with that eye.
Micropthalmia in cats is most often caused by the mother having an active herpes infection during early pregnancy. This is why it is mostly seen in stray, barn, feral, and hoarder's cats, which often aren't vaccinated (Herpes in cats causes upper respiratory iinfection and the 3 in one, or "distemper" shot covers it.)
Squeaky was seized by Animal Control from a hoarding situation as an adult with 4 fat little kittens at her side. She was skin and bones. The kittens were adopted out as soon as they were old enough, but no one wanted Squeaky because of the eye and because she was a "plain vanilla" tabby cat.