How old is my cat when it is 13.5 human years?

To estimate your cat's age in cat years when it is 13.5 human years, you can use the common rule that the first two years of a cat's life equal about 24 human years, and each subsequent year equals roughly 4 human years. Therefore, a 13.5-year-old cat would be approximately 13.5 - 2 = 11.5 years older than 2 cat years, which translates to about 46 cat years. Thus, your cat is roughly 46 years old in cat years.