I just read a brochure on elderly cats that mentioned the "dog years" method of determining age and said it wasn't applicable to cats. They said that a one year old cat is similar to a 16 year old human and a 2 year old cat is similar to a 21 year old human, and after that it's approximately 4(?) "cat years" for every human year - but don't quote me on the last part.

I know there are cat age calculators on the internet, but I don't have the address to any of them off the top of my head. Google could help immensely here I'm sure.

I belive what K & L said is accurate, although I had heard only 3 years for an outdoor only cat. Tubby was 18 1/2 when he died and right now Peanut is 17, but I also know 14 year old cats that have died of old age, so I think 16 is a pretty "life span" age with some living longer and some not living quite as long.