The phrase "dog years" is a familiar one, one that we have probably all used at some point. Many people like to think that there is a specific and general length of time that defines a "dog year," but that is not the case. A dog 's life expectancy is varied according to several different factors. The breed and size of the dog is an important one; a difference in size and breed can mean years in this case. Nutrition, illnesses and injuries suffered, and environment all play a role, as well, in how long you can expect a dog to live.

Large dog breeds like a Saint Bernard have the shortest life expectancy, averaging seven or eight years, while the smallest breeds (such as a Chihuahua) can live to be sixteen years old. Those dogs right in the middle of the size range have a life span that matches. An Australian Shepherd owner could expect to have her pet for ten to thirteen years. There is a tool called the "size or specific breed calculator" that can predict the approximate life expectancy of a dog according to its size and breed.

Those who are familiar with the term "dog years" may also remember an old "rule" that said a dog ages seven years to every one human year. This is not accurate, because of the varied life spans of different breeds and sizes of dogs. Also, the first two years of a dog's life is a period of fast growth and maturing, and would be different than the "year" allowed by the old formula. There is no formula that can calculate any dog's life span in accordance to the human's ageing time-line. There are some similarities, though, between the two species and how they age.

A dog that is one year old, in human years, is an adult dog with fully developed features and a mature mind. In one short year, a dog develops from a puppy into a full-grown adult dog.

There is a study that indicated around 64 percent of all dogs get euthanized before they live out their natural life expectancy. The reasons are to end the suffering of an aged or terminally ill dog, but the result is the same...dogs have a much lower life expectancy due to this than they probably would if they were allowed to live out their lives the way humans with terminal diseases are.

Out of all dogs, roughly only eight percent reach their fifteenth birthday, and sixteen percent are lost to cancer and heart disease each year. Humans are not euthanized when diagnosed with terminal illness, so our life expectancies are more accurately calculated.

Dogs show much the same signs of ageing as humans do, but their symptoms seem to hold off for several years...often hitting them quite suddenly and intensely. Some of the first signs of ageing in canines that you will notice is a decline in the condition of their skin and coat, longer naps and sleeping more, less playful activity, decreased appetite, and loss of teeth. They may also show signs of losing their sight and hearing.

Humans have the same signs and symptoms as dogs, and any other mammals, but we have an advantage in modern medicine and treatments that battle ageing (sometimes for many years past what is natural).

Although there is no exact formula that compares dog years and human ageing in a nice linear way, there are plenty of similarities between us. Those who know dogs can easily spot the signs of ageing in them, just as we can easily point out the signs of ageing in an elderly human.