Cats. Now let’s take a glimpse at a simple formula for calculating feline age in human years. Assume that a 1-year-old cat is equal to a 15-year-old human and a 2-year-old cat is equal to a 24-year-old human. Then add four years for every year after that. (Example: A 4-year-old cat would be 32 in human years.). The following chart shows this formula of calculation: Cats generally live 12 to 15 years, although outdoor cats live an average of only 10 years or so.