Trying to calculate the age of a dog can be difficult. The aging process depends on a dog’s breed and size. Smaller canines have a longer life span than giant breeds. Once a dog has reached his first birthday, he has matured at the same level as a 15 year old human. You can tell by looking at a dog’s graying muzzle that he’s in his golden years. Understanding the developmental stages dogs go through can also help you make a reasonable guess, but unless you were around when he was born or were told his age when you adopted him, you may never know his true age.