A commonly held belief among dog owners is that if they want to know how old their pet is, they simply have to times its age by seven.
Then, it can be decided whether man's best friend is acting up because it is a naughty teenager - or simply due to poor training.
However, new research has found that this method is not based on science, and our pooches may be far 'older' than previously believed.
As people, and animals, age, the number and placement of methyl groups in the genome change. By mapping these, scientists can tell the age of an organism.
The researchers from the University of California San Diego School of Medicine used blood samples from 105 Labrador retrievers to accurately work out how quickly the breed ages.
The study, published in Cell Systems, found the comparison is not a 1:7 ratio over time. Especially when dogs are young, they age rapidly compared to humans. A one-year-old dog is similar to a 30-year-old human. A four-year-old dog is similar to a 52-year-old human. Then by seven years old, dog aging slows, and a 12-year-old dog is 70 in human years.
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"This makes sense when you think about it - after all, a nine-month-old dog can have puppies, so we already knew that the 1:7 ratio wasn't an accurate measure of age," said senior author Dr Trey Ideker, professor at UC San Diego School of Medicine.
Scientists say this new comparison between dog ageing and human could be helpful for vets, so they can work out whether illnesses in dogs are age-related.
The formula provides a new "epigenetic clock," a method for determining the age of a cell, tissue or organism based on a readout of its epigenetics, which are chemical modifications like methylation, which influence which genes are "off" or "on" without altering the inherited genetic code.
Previous studies have found epigenetic clocks for humans, but these don't translate to other species and may not even be the same for other humans.
One limitation of this clock is they only used blood from Labradors, and different breeds are known to live for different amounts of time. Dr Ideker plans to test more breeds, but said that since it's accurate for humans and mice as well as Labrador retrievers, he predicts the clock will apply to all dog breeds.
"I have a six-year-old dog -- she still runs with me, but I'm now realising that she's not as 'young' as I thought she was," Dr Ideker added.
He said dogs are interesting to study because they live so closely with us, perhaps more than any other animal, so a dog's environmental and chemical exposures are very similar to humans, and they receive nearly the same levels of health care.
The research could be useful for humans, not just their pets. The scientists believe the epigenetic clock could be used to test anti-ageing treatments, to see if they had made any difference to the methylation patterns in the genome and therefore altered the 'age' of human cells.
"There are a lot of anti-ageing products out there these days - with wildly varying degrees of scientific support," Dr Ideker said, "But how do you know if a product will truly extend your life without waiting 40 years or so? What if you could instead measure your age-associated methylation patterns before, during and after the intervention to see if it's doing anything?"