Have you heard the popular myth that one human year is equal to seven dog years? This would mean that a dog that is one human year old (365 days) is developmentally about the same as a human child who is seven years old.
Who came up with this idea? We're fairly certain it wasn't a dog. Not only do dogs not wear wristwatches, but most dogs we know seem quite unconcerned with the time. In fact, most dogs we know seem blissfully unaware of whether it's Monday or the weekend!
The idea of a “dog year" probably came about as a result of the fact that dogs develop at a different rate than human beings. A one-year-old child still relies on its parents for nearly everything. A one-year-old dog (in human years), though, does behave much more like an older child.
These developmental differences can be seen throughout a dog's lifetime. If you and a dog are born on the same day, the dog will develop and age much more quickly throughout its lifetime than you will.
For example, when you hit your teenage years, the dog will be quite advanced in age. As you're ready for high school, the dog will be ready for the retirement home! If it helps, you can also think of it this way: a dog's lifespan is a lot shorter than yours (maybe 15 years compared to 75 or more for you), so that means a dog ages much more quickly in the same amount of time.
How old a dog will live depends upon many factors, including its particular breed and size as an adult. Small dogs often live 15 years or more. Medium and large dogs tend to live shorter lives (10-13 years). Giant dog breeds, like mastiffs, might only live 7-8 years.
Given these differences in life expectancy based upon breed and size, you can see that a simple formula of one human year being equal to seven dog years would not be accurate. Although dog experts do not agree on a standard formula, there are some similarities in the ways different people think about things.
The most accurate calculation of dog years must take into account the particular breed and size of the dog in question. You can do research online to find special calculators that will estimate a dog's age based upon its size and breed.
Others use a variable formula that reflects the belief that the first two years of a dog's life are approximately equal to 10-11 years each, with each year thereafter being equal to about four human years. Over time for many dog breeds, this variable formula might average out to the common belief that one human year is equal to about seven dog years.
The variable formula reflects the fact that dogs develop quickly early in their lives. After the first couple of years, most dogs are fully grown and develop more slowly thereafter. The variable formula still does not take into account particular breed and size information, though.
Dog experts know that some breeds tend to live longer than others. As mentioned before, a dog's adult size is also an important factor. If you take these factors into account, a sliding scale for each dog is the only accurate way to estimate a dog's age in human years.