On average, a dog year is the equivalent of seven human years; however, dogs age much faster in the first couple of years of their lives. A dog's first year is actually equivalent to about 15 human years.
The size and breed of the dog can also influence how quickly the dog ages. Dogs under 20 pounds, tend to live longer than larger breeds; however, they also tend to mature faster than their larger counterparts. By the time dogs are around 5, medium breeds (between 21 and 50 pounds) and larger breeds (over 50 pounds) start aging more quickly than smaller breeds.