The often-quoted ratio of 1 cat-year being 7 human-years (for dogs as well as cats) is a silly old myth which was invented by people who are not very good at arithmetic. According to the Guinness Book of Animals, the actuarial equivalent of 1 year (dogs) is 16 years (human); 2 is 23; 3 is 28, and every further year is about 4. So a dog aged 10 is equivalent to a human aged 56, and 15 is equivalent to 76. This is based on the physical development of a dog, and how many dogs live to each age. Hence it is very common for silly people proudly to proclaim that their 15-year-old dog is the equivalent of 105, even though it's only 76. I do not know the specific statistics for cats, except that they tend to live slightly longer than dogs.