Although that could be used as a very rough guide, it ends up being misleading, especially when used to calculate younger or older dogs.
Figuring out how old your dog is relative to human age isn’t as simple as people think. A number of factors come into play. The first factor is a dog’s size and/or breed. For instance, a Bulldog’s life expectancy is estimated to be around 7 years, and Great Dane lives to be 8.5 years on average. Compare that to a Miniature Poodle who’s average lifespan is close to 15 years. So the breed makes an enormous difference in the number of years a dog will live.
The world’s oldest dog, which died aged 21 years in August 2009, has been described as 147 dog years old.
Canine aging is much more rapid during the first 2 years of a dog’s life. After the first 2 years the ratio settles down to 5 to 1 for small and medium breeds. For large breeds the rate is 6 to 1, and for giant breeds the rate is 7 to 1. Thus, at 10 years of age a Great Dane would be 80 years old while a pug would only be 64.
So many factor to consider when determining the actual age. I suppose that’s why the 7 year myth came in to play. Regardless of the “human” age, the unfairness of it all is that our pet’s life cycles are too short and that sucks! Plain and simple.