How old is that doggie in the window?

You've probably heard all about "dog years," plus the rule of thumb that one dog year is equivalent to seven human years. And it's a cute thought, right? It's fun to think that your 2-year-old pupper is almost as old as you – or that your 14-year-old family friend is actually almost 100.

But dig a little deeper and that rule kinda falls apart. After all, puppies often go through their rebellious "teen years" before they're a year old. And some dog breeds live much longer than others. So a 7-year-old Jack Russell terrier – estimated to live 13 to 16 years – should not be the same "dog age" as a 7-year-old German shepherd, which has an average lifespan of just seven to 10 years.

And that's because, well, the seven-year rule isn't really based on science. In fact, there was no hard and fast rule for determining your puppy's age in "human years."

At least, until now.

Yup, scientists have finally figured out how to truly tell your dog's age. And, surprisingly, it all relies on genetics. Here's how it works.

When scientists went looking for the real way to gauge a dog's age, they went straight to the source: DNA. But the best way to age a dog isn't looking at their genetic makeup – except for random mutations that can accumulate throughout a dog's life, the genes themselves don't change all that much.

Instead, the scientists looked at how active the genes are, and how that changes as the dogs got older. And that's where epigenetics comes in. In case you didn't know, epigenetics is the study of gene activity. It tracks whether certain genes are more or less active. Some genes might become more active as you age, while others become silenced, or "turned off" – so it's almost like they no longer exist.

The researchers studying dog aging looked at a genetic modification called gene methylation. It's exactly what it sounds like: a small chemical, called a methyl group, attaches the DNA, and that gene is then considered methylated. Methylation lowers gene activity, which means it silences genes, so that they're no longer expressed and used to make protein.

Methylation tends to be more common as you age. Scientists already know that's how it works in humans – as you get older, your genes become more and more methylated – and it occurs in other mammals, like wolves, too.

And it turns out, the same thing happens with dogs. To check, researchers scanned the genomes of 104 Labrador retrievers of various ages, to see how their DNA methylation levels corresponded to their age. They found out at dog methylation levels follow a similar trajectory to human ones.

Since the researchers now had a timeline for DNA methylation as dogs age, they can compare it to the levels of methylation in people – and find out how "dog years" and "human years" really line up.

Overall, they found that dogs typically reach puberty at about 10 months – so your 1-year-old puppy is really a "teenager." And they also found a formula for calculating any dog's age using a natural logarithm:

• Dog's age in human years = 16 ln(dog age) + 31

That means a 3-month-old puppy, just ready to bring home, is equivalent to 8 years old. And 10-year-old Fido is really about 67. And if your doggo is 14 years old? That translates to about 73 years old in human years.

So what's next in canine research? Well, the study looked at dogs of one breed, so it's possible that certain breeds genetically age faster than others. The study authors have launched a Dog Aging Project, that collects data on all breeds, to learn more about how it works.

In the meantime, though, use the formula above to estimate how old your fluffy best friend really is. Here's hoping you'll be spry enough to catch a frisbee at 73, too!