The old rule that a dog’s age is equivalent to 7 years of human life is not quite accurate. Dogs in their first year age much more quickly than a human child of one year. At one year a dog’s age is similar to that of a human, 15 years old. Different breeds and different size dogs age differently. Small breeds are generally longer-lived than large or giant breeds.
As an example, a 20 lb. or less adult dog may age 4 human years for every year of life over the age of 2. But adult dogs weighing over 90 lbs. may age 7 human years for every year of life over the age of 2. Dogs that weigh over 20 lbs. but less than 90 lbs. average 4, 5 or 6 human years for every year of life. The higher the weight of a dog, calculate the higher number of years.
So taking into account your dog’s breed and especially weight, you can calculate his approximate age.