Q. I’ve read that “one year equals seven” isn’t true anymore for dogs. What’s the new guideline? And how do you figure out how old a cat is?
A. The old “dog years” formula (one year of a dog's life is equal to seven years of human life) never really worked for any dog, based on a comparison of the general lifespan of humans vs. dogs. Cats age at a more predictable rate than dogs do, mostly because we don’t see the range of size and body types we do in dogs, but the 1-to-7 formula doesn't work for them either.
How Old Is My Dog?
Consider two 10-year-old dogs, a Great Dane and a Chihuahua. Under the 1-equals-7 formula, they’d both be considered the equivalent of a 70-year-old person. And while that’s probably not too far off the mark for the Great Dane (10 years for a Dane is indeed near the outer edge of life expectancy), the Chihuahua is likely still acting middle-aged.
But even the Great Dane didn’t match the old formula when he was young. In the first year of his life, he would have attained most of his height, something no human 7-year-old ever does.
It’s more accurate to think of the first year of a dog’s life getting him just over the first big jump of adolescence, with the second year bringing him close to full adult maturity physically, although as in humans, mental maturity may still be on the horizon. From there, it really does depend on the kind of dog in question. I think it’s more important to focus on the individual dog, with the knowledge that size and breeding matter when it comes to aging. Roughly put, you can consider your dog to be a senior citizen at 7 if he’s that Great Dane, but not until 11-plus if he's the Chihuahua.
