For the longest time, it was thought that seven human years was roughly equivalent to one dog year. The ratio becomes problematic when you think about pups that live to 18 years of age or older, since no human is known to have lived beyond 122.
Dog people know that, just as there is great variation between breeds, there is a wide spectrum of typical life expectancy. Body size is generally correlated with life span in mammals (so, for example, a larger mammal, like an elephant, will have a longer life expectancy than a smaller mammal, such as a mouse). But what's interesting is that the opposite holds true for dogs--even though small dogs reach maturity faster than big dogs.
Basically this means small dogs age more quickly than big dogs in the first couple of years, but slower once they hit adulthood. So at two human years, a small dog is older than a big dog, but younger at five years.
According to Kate Creevy, an assistant professor of internal medicine at the University of Georgia, there aren't any other species that have the same degree of size diversity within a single species as dogs do. This could be the key to the ageing phenomenon. Kate believes that if we genetically engineered a cow weighing 20 pounds at adulthood and another weighing 2,000 pounds, the same thing could happen.
But why does size matter? One of Kate's colleagues, Daniel Promislow, a professor of genetics, believes that the aging phenomenon is linked to cancer. Large dogs have a 50% chance of dying of cancer while small dogs only have a 10% chance.
No one seems to know where the seven dog years to one human year equation came about. It first appeared in math text books in the 1960's asking children to calculate a dog's age using the 7:1 ratio.
If you did factor the varying rates of ageing and differing life expediencies, a more accurate estimate across all breeds would be six dog years to one human year. So the seven years wasn't far off.
However, scientists have come up with a more accurate way to calculate your dog's age, which takes size and breed into consideration:
Years Per Human Year - For the First Two Years:
Small dogs: 12.5 years
Medium dogs: 10.5 years
Large dogs: 9 years
Years Per Human Year - For Three + Years
Small: Dachshund (Miniature) 4.32, Border Terrier 4.47, Lhasa Apso 4.49, Shih Tzu 4.78, Whippet Medium 5.30, Chihuahua 4.87, West Highland White Terrier 4.96, Beagle 5.20, Miniature Schnauzer 5.46, Spaniel (Cocker) 5.55, Cavalier King Charles 5.77, Pug 5.95, French Bulldog 7.65
Medium: Spaniel 5.46, Retriever (Labrador) 5.74, Golden Retriever 5.74, Staffordshire Bull Terrier 5.33, Bulldog 13.42
Large: German Shepherd 7.84, Boxer 8.90
You can see how the ratio varies significantly after the first two years. According to this model, a Bulldog will age an average of 13 years per human year whereas a Miniature Dachshund only ages about four years.
Unfortunately they only calculated the 3+ year ratio for a handful of dog breeds, but this means that my 1-year old Border Collie, Scuttle, is about 10.5 in human years!