Thanks, Wade -- I know you know about cats.

She was ~ 16.8 years old; counting my time with her in Rent-A-Cat status -- the neighbors' cat with issues with their toddler, of the usual kind -- I had her companionship for 6+ years.
This is 'average' for US cats, so many being on cheapest supermarket bargain-kibbles with trash inside. On a decent diet she should have reached mid-twenties.

My 'decent diet' since late '05 could not undo the earlier dry-food-only diet, neglected dental cleaning (some cats drink enough; most do not -- Sierra was one of those, as I observed her water use.)
Still a guess though, as she had wet people-grade food here and so needed less additional water ie. my 'data' isn't necessarily relevant. A nasty incitement of those awful What-If?s, though..

(A refuge for old or dying animals of many species -- the rich pay more for lifetime-care there, to support their considerable pro-bono works) -- a 34 yo cat died recently; last week a 24 yo.

They eschew allopathic meds except in certain specific cases (where they actually work and do not create insurmountable Other problems) and they feed a very well-tailored healthy diet to each species; not a single thing from the supermarket. Etc.

They deem that a 'normal' cat lifetime is more like mid-twenties -- and have the stats to show how this might be correct.

And yes I'm aware of the attraction of running water for many -- as many owners never even discover.
Many but not all: I tried a small fountain with Sierra; she ignored it. Others would make a fetish of visiting for a sip several times a day. As I said: she Would not have drunk enough; this opinion is echoed by the Vet who wrote Your Cat; she has practised 28 years, is an attorney and ... at one point was at executive level in a 'major' pet food corporation (Purina? -- not revealed) and she avers that, many many cats live a life of incipient dehydration -- unless fed complementary wet food, preferably of actually nourishing quality. That means: sans carbs they cannot process/do not need and the garbage within slaughter-house debris (much of which should have been burned) == common in US pet foods, even some expensive ones.

Yes, Sierra died at the 'average' age -- but well in advance of what might have been. Causality cannot ever be found now, in any event.
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Squeak (outside cat) gets healthier weekly and shall profit from this-all experience and delving. He actually looks sleek now and knows it: lots of grooming. No prior history on him, but he was pretty scruffy, had untreated ear mites, worms etc. as he sat crying in the rain in 3/08. Decent care is as rewarding to the onlooker as to the subject, I see.