Interestingly, the old equation for comparing human years to dog years which seems to have been accepted as "common knowledge" is not really accurate. That is, it was commonly expressed that a year in a human's life roughly equates to seven years for your dog. Of course a more thorough explanation will include the size of the dog, the breed, the diet, sex (male or female), alteration (neutering adds to longevity), lifestyle, health care, and more.
Unfortunately though, as a rule the situation is, on balance, a little worse than the old one-to-seven ratio guesstimate for dogs. Cats have a wide variation in life expectancies but according to About.com the average is twelve to fifteen years. Another reason the 1 to 7 ratio is a myth is that different breeds of cats and dogs mature at differing rates. Some larger breed dogs can take a couple of years to mature while some small breeds are considered mature even prior to one year.
Follow this link for an excellent graphic representation of comparing human versus dog years.
Consider for example a Siberian Husky which is obviously a medium size dog (around fifty pounds for an average adult male). At the age of two the dog will actually be situated in his relative-to-humans-age of early to mid-twenties. Now it is known that, generally speaking, the larger the breed of dog the shorter the lifespan. A good article from WebMD: Which breeds live the longest. Smaller mixed breed dogs can live several years longer than their larger relatives.
Bottom line: life is short, even shorter for your animal so cherish him or her while you have a chance. Go out more and spoil them a little more. Inevitably we all will eventually lose our pets but we can make changes now so that the memories are even sweeter! Make sure to take plenty of pictures and videos too.