I always wonder about these cases. I know I have seen a 30 year old I euthanised him and as a practice we had documentary evidence of castrating him at 6 months of age. But neither the owner nor I felt like rushing to the papers to report his age. I suspect he is not the only cat of this age around, but the rest prefer to live in anonymity.
He looks very like the old cat who roams our neighbourhood (he does have a home, just seems to spend all his time outside). Except Eddie's hair seems to be falling out recently. We were told he was about 20 ages ago - I must ask his owner how old he really is.
I fostered out two cats to a friend of mine when they came into us after their owner died. They had all the paperwork going back to when they were kittens. They were 23 and 25 when they came into us and they both lived with my friend for a further two years.