What's a cat's lifespan, and how do cat years convert to human years?

Quick answer

The average lifespan for an indoor cat is about 15–20 years. Year 1 of a cat's life is roughly equivalent to 15 human years, year 2 to 24 human years, and after that, add about 4 human years for every additional cat year. From age 7 onward, cats enter their 'senior' stage, so an annual checkup is a good idea to catch major health issues early.