My wife was told by a vet that a realistic way to figure the age of domestic cats was like so:

First six months of a cat's life = first fifteen years of a human's

Second six months of cat's life = next five years of a human's, i. e. a one-year old cat is the equivalent of a twenty year old human.

After that, every year of the cat's life = four years of a human's, i. e. a five year old cat is equal to a 20 + 16, or 36 year old human.

A ten year old cat would be equal to a 20 + (9 x 4), or 20 + 36, or 56 year old human.

At 17, a cat would be equivalent to a 20 + 64, or 84 year old human.

Or so I figure it.

Peter.