The guideline of one dog year equalling seven human years is a persistent myth. Not only is the widespread rule of thumb incorrect, it is more inaccurate as a dog ages. In reality, a dog's equivalent human age depends on multiple factors, including a dog's size.
While it is true that dogs physically mature more quickly than human beings, their growth starts with a large spurt and then begins to taper off. Many 2-year-old dogs are the equivalent of 21 in human years, but then they begin to age an average of four human years every year after that.
Dogs weighing fewer than 30 pounds tend to live longer, often into their teens, than heavier dogs. On the other hand, dogs weighing 100 pounds or more are generally regarded as relatively old when they are 6 or 7 years old.
Veterinary doctors have long known that the seven-year dog-to-human ratio is inaccurate. However, the public in general has long attempted to find a simple way to compare the two species' lifespans. At Westminster Abbey, a 13th century inscription indicates that the ratio is nine-to-one.
No one seems to know when the seven-to-one ratio started, but many animal scientists point out that it doesn't actually make sense, as it would mean human beings would be able to reproduce at the age of 7, for example. Small dogs mature even more quickly than larger breeds, further skewing seven-to-one ratio logic.