How Old is Your Jack, Really?

a jack russell terrier lying down, facing to the right

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At the dog park, one of the first things pet parents ask one another is the name and age of one another’s dogs. The name is easy. Age seems pretty straightforward, too. How many years since your doggo’s birth?

But a five-year-old dog is a lot different from a five-year-old human. And different breeds seem to age much faster than others. On top of that, some dogs classified as “seniors” still have as much pep and vitality as a pup.

So people have come up with different systems for counting a dog’s age. You might be familiar with the idea of “dog years,” but there are actually quite a few other ways for measuring a dog’s age.

Are you curious?

Puppy, Adult, Senior

The broadest classification divides a dog’s lifespan into three parts: puppy, adult, and senior.

Experts generally agree that the “puppy” stage lasts between six and eighteen months. That’s quite a range! Why is that? Well, small dogs tend to mature faster than large dogs. That means your large-breed pup may act like a puppy for almost two years, while your Jack Russell may be ready for serious doggy business well before that.

Adulthood can begin anywhere from between one and three years. And what about senior dogs?

Well, just as large breed dogs mature slower, they also age faster. So for some large breed dogs, the golden years start around age five. (Many Humane Society branches use age five or six as the baseline in their Seniors for Seniors programs).

But for a “fun sized” dog like a Jack Russell, age five is just getting started. Many don’t consider a Jack to be a senior dog until age ten or even older.

There has to be a better system.

Dog Years

A lot of us are familiar with the idea of “dog years.” The common wisdom is that one dog year is equal to seven human years. And sometimes that works.

It’s not the most precise system, though, and it doesn’t take into account individual, size, and breed differences.

Another system proposes counting the first year of a dog’s life as 15 human years. The second year is nine human years. And every year after that, depending on a dog’s size, counts as between four and six years — except when it comes to big dogs. When large breeds reach age 15, this system counts the 15th year as 27 human years!

But who wants to keep going to a chart every time they want to calculate a dog’s age?

The Latest Scoop

Scientists at the University of California, San Diego have come up with a system that looks at physical age in terms of DNA methylation.

What on earth does that mean?

Well, over the course of a person’s — or an animal’s — lifetime, chemical changes to that organism’s DNA leave traces. And those traces can be read as a sort of biological clock.

DNA also shows the effects of lifestyle, disease, and various genetic factors on our bodies. And these things are factored into the age determination as well.

And scientists believe that an organism’s DNA methylation status is a good way to determine both physical age and life expectancy.

But what’s an ordinary person without access to DNA analysis equipment supposed to do with that?

The numbers

Fortunately, we don’t have to do anything with that.

In the course of the study, the scientists scanned the genomes of 104 Labrador retrievers between the ages of six months and 14 years. Then they compared the DNA of dogs across that age span with humans across the human lifespan.

They found that age-related DNA methylation happened similarly between the two species. And from there, they were able to come up with another equation that compares human years to dog years.

It’s a complicated equation.


First, multiply the natural logarithm of your dog’s age in years by 16. Now add 31 to that.

Or, you could just use this handy calculator.

Limitations of the study

The study doesn’t seem to address the difference in lifespans between large dogs and small dogs. Also, the dog genome sample only came from one breed — the Labrador retriever.

Perhaps the Lab, being of medium size and having a medium lifespan of 12 years, provides a good average. It would be interesting, though, to see an expanded study that looks at other breeds and other sizes of dog, as well.

What About the Jack Russell?

The Jack Russell, being a small dog, matures fast and ages slow. The best of both worlds, wouldn’t you say? That means they develop an adult dog’s wisdom early — between six and twelve months. They also have a very long lifespan — 13 to 16 years. On top of that, they’re generally an extremely healthy breed.

So how old is your Jack?

If they’re under a year, they’re a pup!

Jacks between the ages of one and ten are energetic adults.

A Jack Russell over the age of ten may be considered a senior dog, but they might still be in the first half of their lifespan! After all, Meg, a Jack Russell Terrier in Britain, was crowned Britain’s Oldest Living Dog at the age of 25!

Increase Your Dog’s Lifespan

Naturally, we all want our dogs to live as long as possible. So, how can you do that?

Obviously, see your vet regularly and make sure your pup is current on all of his or her vaccinations.

Also, feed them a balanced, nutritious diet, and make sure their treats are healthy ones.

Make sure your dog gets plenty of mental and physical exercise. And help them to maintain a healthy weight.

And if your best bud is getting up there in years, take some time to spoil them rotten!

How old is your Jack? Do they still give you a run for your money?

Featured Image: CC BY-SA 2.0 by Andrew Kuchling via Flickr

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