My parent's cat will be 18 years old, in the next month or so. We got her the beginning of Oct 1992, when she was about 4 months old. I was still in high school, so she was my cat for a couple years. My Dad still calls her my cat, even though, at this point, she is most definitely HIS cat.
The vet says she is doing remarkably well, and acts a few years younger than her age. She does have a couple issues. Her sense of taste/smell is going so she will only eat wet food now. And they have to change it every week or so because she will suddenly stop eating a certain type. But don't give it to her for a month or two, and she will eat it again. And she's not litter box reliable anymore. When my parents moved to FL, she went from a 3000 sq ft house to a 600 sq ft apartment, to a 3000 sq ft house when my parents sold their first house, and bought in FL. She, apparently, preferred the short walk to the litter box and is too "lazy" to walk far for one now. They took her to the vet, and after running a bunch of tests, that what he said...she's lazy. So now they have multiple boxes, but sometimes they have to lock her up in their master suite.