I know alot of people that have cats well above 20 years old.
Alot will depend if they're indoor, outdoor or indoor/outdoor cats.
I have to say on the average, mine usually lasted 14 years old. My one cat was 19 when he was put down. He was very spry & spunky up to about the last few weeks. He became real forgetful, confused, & started dropping weight real fast.
Personally....I have NEVER seen a cat just die from old age. They seem to keep on going & will suffer if they're hurting for a long time.
Everyone I've ever had (including all my family members & friends cats) had to be put down in the end.