Cats age rapidly during the first two years of life, with the first year equivalent to 15 human years and the second year the same as 10 human years. After that, one cat year equals roughly four human years. Charts are available on websites such as CatChannel.com and Purina.
In order to survive in the wild, cats had to mature rapidly. A female cat can reproduce at six months, even before her bones stop growing. That's why veterinarians recommend spaying, and neutering, before a kitten reaches that age. After that two-year growth spurt, the age process settles down for the remainder of a cat's life.
One website, CalculatorCat.com, breaks the age comparison down even further. On the bottom left of the webpage is a Cat Years Calculator. By entering a cat's age in years, or months, it will calculate the human year equivalent. Change the entry from "My cat is" to "I am" and add a human age and the cat equivalent will appear.
The site also shows that the cat vs. human years comparison is not an exact science. A posted chart, taken from Tracie Hotchner's, "Cat Bible," notes that a one-year old cat is equivalent to an 18-year old human.