Cats age much faster in their early years compared to humans. The first year of a cat's life is roughly equivalent to 15 human years. The second year adds about 9 more human years, making two cat years approximately equal to 24 human years. After that, each additional cat year is roughly equal to 4 human years. For example, a 6-year-old cat would be about 40 human years old (15 + 9 + 4*4) and a 10-year-old cat would be about 56 human years old.
This calculation reflects the rapid maturation cats undergo in their early life and then a slower aging process as they grow older. Indoor cats often live longer and age slower in human equivalent years than outdoor ones, but the formula above is a good general guide.