According to PawNation, the idea that dogs age 7 years for every 1 human year is a myth. Not all dogs age at the same rate, so the actual method of determining a dog's age compared to a human is complicated.
PawNation explains that breed is one major factor in the rate at which a dog ages. Among mammalian animals, dogs are the most diverse. Some dogs are smaller than housecats while others are taller than the average human when they stand on their hind legs. Smaller dogs have longer life expectancies than larger breeds, which is why calculating a dog's equivalent human age is no easy feat. For example, a chihuahua ages less every year than a great Dane.
WebMD also explains the truth about comparing a dog's age to a human's. Because dogs mature more rapidly than humans, a dog's first year of life is actually equivalent to 15 human years, not 7. At 2 years of age, a dog is like a 24-year-old human instead of a 14-year-old as was previously assumed. Some large breeds are considered elderly at 5 years of age while many small breeds are not seniors until they are around 10 years old.