When people ask you how old your dog is, do you multiply the numbers of years you’ve had Rover and multiply by 7? Most people do. Well, that isn’t correct. More factors come in to play in determining your dog’s age than just comparing it to human years. Breed size is the biggest. For dogs, the first human year of growth is the quickest, aging the equivalent of 20 years in just 365 days. Then breed takes over as larger dogs age more rapidly. With cats, it’s a steadier climb, reaching maturity by its seventh year on Earth.