To convert a cat's actual age to his comparable age in human terms, allow 15 years for the first year of life, add nine for the second year, and add four more for each subsequent year. For example, a 5-year-old cat is approximately equal to a 36-year-old human.
As with humans, the life expectancy of any particular cat depends on the health of the individual. A cat that eats a healthy diet and gets sufficient exercise usually outlives one who becomes obese. While the average lifespan for indoor cats is about 15 years, it isn't unusual for cats to live for 20 years or more. According to Cat World, a cat named Creme Puff of Austin, Texas, lived for 38 years before dying in 2005. Creme Puff's comparable human age was 168 years old.