To determine a cat's age in human years, one must look at where the cat stands developmentally, according to Catster. During its first year of life, a cat will reach the same stage of development as a 15-year-old human. During year two, the cat grows to the equivalent of 24-years-old.
According to Catster, every year a cat lives after the second year is equivalent to four human years developmentally. These numbers are for domesticated indoor cats only, as outdoor cats tend to age much more quickly. Outdoor cats also have shorter lifespans, averaging only 10 years, compared to an indoor cat's average lifespan of 12 to 18 years.