There are many myths that float around about dogs, one of the biggest being about the way dog's age in comparison to humans. It has been noted that for every year a dog is alive, it equivalents to 7 human years. This is inaccurate! It is true that dogs age much faster than humans, but the rate in which they age is dependant on their size. Small dogs (<20 pounds) age the slowest and large dogs (>90 pounds) age the quickest.
Every dog is considered a senior by the age of 7-8 years old, but the difference in human years between a small and large or giant breed is significant. Therefore a giant breeds life-span is said to be much shorter than one of a small breed.
Below you can see the comparison between small, medium, large and giant breeds aging vs humans.
Source (Image): Science