The basic premise is that each calendar year a dog lives is equivalent to seven human years at each stage of a dog’s life. But a new study shows that things are not so simple

If your dog has been alive and kicking his paws for about a decade, the popular opinion is that he has aged like a human would at the respectable age of 70. This conversion factor – each year of a dog’s life is considered seven human years – is obtained by dividing the human life expectancy – environments by 77 – in the canine life expectancy – environments 11.

For example, most dog breeds reach sexual maturity between the ages of six and 12 months – the upper limit of this range corresponds, according to the traditional conversion, to the human age of seven. And at the other end of the spectrum, although it’s not common, there are dogs that are known to have lived more than 20 years. According to the “factor of seven” conversion rule, this would be equivalent to the eternal age of 140 human years.

And to complicate matters further, the life expectancy of dogs depends significantly on the breed. Smaller dogs tend to live significantly longer, and what’s more, they age more slowly than larger dogs.

All this raises the question of what exactly we mean when we say age. The most obvious way to describe it is simply the amount of time that has passed since birth. This is known as the chronological definition of age.

But there are also other descriptions. “Biological age”, for example, is a more subjective definition, which depends on the assessment of physiological indicators to identify a person’s development. This includes indices such as the “exhaustion index” – surveys that take into account the person’s disease state, cognitive impairment and activity levels.

And there are more objective biological markers for aging, such as gene expression levels (genes produce proteins at different rates in different life stages) or the number of immune cells. The rate at which biological age increases depends on genetic hereditary factors, mental health and lifestyle.

For example, if you spent a lot of time eating junk food and smoking cigarettes instead of exercising and eating healthy, chances are that your biological age will exceed your chronological age. Or maybe you are 60 years old with a 40 year old body if you have taken good care of yourself.

A new look at a dog’s life

When it comes to comparing the age of animals of different species, the biological definitions of age are much more useful than the chronological equivalents. Knowing that a hamster is six weeks old does not give good information about the life stage of that animal even if you know that a hamster’s lifespan is only three years. Knowing that a hamster has reached an age where it can breed is much better information about its maturity level.

The authors of the new study on aging say that a logical way to measure biological age is through so-called “epigenetic clocks” – changes in the packaging of our DNA that accumulate over time in all mammals.

In particular, “methylation”—the addition of methyl groups (a carbon atom bonded to three hydrogen atoms) to DNA—seems to be a good indicator of age. Many prominent physiological markers, such as the development of teeth, appear to show the same levels of methylation in different species. So by matching methylation levels in Labradors and humans, the researchers derived a formula for mapping the dog’s age to its human equivalent.

The formula is: human equivalent age = 16 x ln(dog’s chronological age) + 31.

In the graph below, you can see the operation of the natural logarithm in the conversion of the years a dog has lived (the age of the dog) to the equivalent human age in the dotted red curve. The curve shows that dogs reach maturity very quickly at first, but then their aging slows down, meaning that they experience most of their lives as a kind of continuous middle age.

A useful shortcut is to remember that a dog’s first year counts as 31 human years. Then every time the dog’s chronological age is doubled, the number of equivalent human years increases by 11. So eight calendar years are three “multiplications” (from one to two, from two to four and then from four to eight) and the age of a dog is obtained which is equivalent to 64 (31 + 3×11).

This useful approximation is plotted as the black curve in the conversion figure below. The green line represents the factor of seven rule whose reliability has been undermined and unrealistic ages at the upper end of the dogs’ age range emerge from it.