Calendar years, so if they say the dog is 2 years old, it was born in 2015. These ages are usually an educated guess. Advice on deciding on an age is look at your life and why you want a dog. Young dogs and puppies may not be trained or socialized well, they will take a lot of work. Older dogs are usually house trained, and set in their ways. Young dogs make great "blank slates" to train your way, but take a lot of work. Older dogs might come with a couple bad habits but make great companion dogs. Depending on breed they can be excellent protectors.
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I am not really sure how this works but a lot of veterinarians claim that 1 human year is equivalent to 7 dog years. To me this is very complicated and hard to comprehend.
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