It's a myth that's been perpetuated for years, one that animal scientists have been working hard to dispel. But it's been accepted by the general public for so long that it's extremely hard to reverse what seems to be common knowledge.
It is true that dogs age at different rates than humans, but the seven to one ratio is believed to have been a marketing ploy. William Fortney, a veterinarian at Kansas State University, told The Wall Street Journalthat he thinks the myth was created as "a way to educate the public on how fast a dog ages compared to a human, predominantly from a health standpoint. It was a way to encourage owners to bring in their pets at least once a year."
Business Insider points out that "if humans aged seven times slower than dogs, then many of us would be able to reproduce at age 7 and live to be 150. Obviously that's not the case." That would be a little insane.
Dogs reach sexual maturity in their first year of life, so that's equal to between 12 and 15 human years, not seven. According to AlterNet, "By age 2, dogs are fully mature, and old enough to vote, not a right you'd want to extend to human toddlers."
Not to mention that "compared to humans, dogs age more quickly at the beginning of their lives and slower toward the end. Therefore, calculating your dog's age relative to yours is a bit tricky, but luckily it's possible." (Business Insider)
There's a chart that more accurately estimates the human-equivalent age of your dog based on their size and age. You can see that while small dogs mature faster, their aging slows as they get older when compared to big dogs, who age more quickly and thus have shorter lifespans.
It's still a mystery to scientists why bigger dogs have shorter lifespans. In animal species, generally larger members of the species live longer. But, no matter the size of your dog, the most important thing is to love him or her with all your heart, for all their years!