It’s pretty well known that for a medium-size dog, every year is equivalent to about seven human years. But what about cats?
Their rate of aging changes over time. At the end of a cat’s first year, it is about the same developmentally as a person who is 15. At the end of its second year, it is at the same place, more or less, as a 24-year-old human. After that, a cat ages roughly four people years for every calendar year. A 10-year-old cat, for example, is at about the same place in its lifespan as a 56-year-old human: 24 for the first two years plus four per year for the eight subsequent years, or an additional 32.