Cats mature faster than dogs do. The first year of a cat's life equals 24 human years, while the first year of a dog's life equals 15 human years. After the first year of a cat's life, one cat year equals four human years; dog years are more variable.
Size affects canine aging, so while small breeds age more slowly than cats, large breeds age more rapidly. At the age of 10, a cat is equivalent to a 60-year-old human, according to CatChannel.com. A toy dog of the same age is equivalent to a 56-year-old human, a medium-sized dog is equivalent to a 60-year-old human, and a large dog is equivalent to a 66-year-old human, according to WebMD.