The first year of a cat's life is equal to 15 to 24 human years, and the second year is equal to four to nine human years. After that point, one cat year equals four human years.
During the first two years of life, a cat ages rapidly compared to a human being. A 6-month-old female cat may be able to reproduce, equating to a human in her early teens. A cat is fully grown when its bones stop growing at between 1 and 2 years of age, a level of physical maturity that humans reach at age 24. The larger the cat is projected to be at maturity, the longer its skeleton takes to reach full growth. Cats kept indoors usually live 12 to 18 years. Cats living outside have a shorter life expectancy due to greater risks of accident and illness.