Conventional wisdom holds that one year in a dog's life is equivalent to seven in a human's. However, while this number is not entirely inaccurate over a dog's lifetime, a dog ages faster initially. A dog's equivalent age also depends on its size and breed.
Small dogs live longer but age faster than larger dogs. Most breeds of dogs are roughly equivalent to a human 15-year old at 1 year old, with larger discrepancies appearing after the dog turns 5 or 6 years old. A large dog weighing over 50 pounds may be roughly equivalent to a 70-year old human after turning 10 years old, but a small dog weighing less than 20 pounds may only be middle-aged. Medium-sized dogs hover roughly in between the two extremes.
A dog's age can be determined by examining its teeth for growth, wear and tartar. However, dental care has a significant impact on a dog's teeth, though a dog that shows no signs of yellowing is most likely a puppy under a year old. By ages 10 to 15, a dog may be missing some teeth, and the remaining teeth are worn with heavy tartar build-up. A dog's age can also be calculated through a full physical exam.