Dogs' early development is much faster than humans.
Between 6 and 8 months a dog is an adolescent, not the equivalent of 4 years, and can reproduce at one year.
The calculation works later on.
At 7-8 years the physical age of a dog is equivalent to 50-55 human years and at 15 years is a centenarian in human terms.
So the x7 formula works best for dogs over 5 years old.
Another factor is size. Canine longevity is inversely proportional to the size. Smaller breeds live longer than big breeds.
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