Compare Human Age to Dog Age
The common unit of measure to compare dog versus human age is:
1 human year = 7 dog years
Different breeds of dogs age at vastly different rates so this rule of thumb only goes so far. Giant dogs like Great Danes and St. Bernards may have a life expectancy of under 10 years. Chihuahuas might still be going strong at 15. You can think of some dogs as either aging faster or simply not living as long as others. In any case, a life that is significantly shorter than the average human life is one of the difficulties that dog lovers face.
Alternatively, since dogs reach maturity in about 1 year, that could be seen as 16 in human terms. Then add about 7, 5 or 4 years for every year your dog ages depending on the size of your dog.
Giant breeds examples: Great Danes, Mastiffs Large breeds: Retrievers, Collies Small breeds: Chihuahuas, Pomeranians
A 5 year old giant breed dog would then be: (5-1) x 7 +16 = 44 in human terms
A 5 year old large breed dog would then be: (5-1) x 5 + 16 = 36 in human terms
A 5 year old small breed dog would then be: (5-1) x 4 + 16 = 32 in human terms
Specific breeds such as Border Collies and Bulldogs can have shorter life expectancies than their size would suggest. This could be because Border Collies lead hyper active lives and Bulldogs are essentially dwarfed Mastiffs and share the shorter lives of their ancestors. Beyond genetics, good nutrition, daily exercise, medical care, a warm family environment and an interesting life all serve to increase the lifespan of your dog. Note that the average lifespan of a dog in the US has doubled in the last several decades. The basic dog hasn’t changed as much but the care of dogs has changed dramatically for the better.