So at the end of her first year, your cat is approximately at the same developmental stage as a human that is 10 or 15 years old, and by age two, your cat will be around 25 in human years. “After that, cats tend to develop more slowly,” Barrack said, aging about four or five human years every 12 months. Putting your pet’s age in human terms not only helps you empathize with your cat, it also show the importance of regular veterinary checkups.