It’s no secret that kittens grow rapidly but things typically taper off around 12 months. When it comes to functional age, however, your cat will still progress quickly for another year or so. If you were to compare a cat’s aging rate to that of a human’s, your cat will be similar to a 15-year-old human when he reaches his first birthday. He’ll be in his mid-20s by the time he reaches 2 years old but, after that, each cat year is equivalent to about 4 human years.