Image via iStock.com/Kkolosov By Deidre Grieves When it comes to understanding how a dog ages, you may have heard that one dog year is equivalent to seven human years. But according to Dr. Lisa Lippman, a veterinarian based in New York City, that isn’t an exact calculation for determining dog age. “The ‘seven-year rule’ is a simplified explanation of canine-human aging,” she says. According to Dr. Lippman, a medium-size dog that’s well cared for will live roughly 1/7th as long as their owner, but different breeds of dogs age differently. This guide will explain how a dog ages and how to best care for your dog at every life stage. Dogs Age Based on Size and Breed Dr. Lippman explains that the dog-years-to-human-years equation is actually more about weight than it is about age. “A 5-year-old dog that weighs 20 pounds or less is about 33 ‘human-years-old,’ where a dog that weighs over 90 pounds is closer to 41 years old in human years,” she says. The dog age ranges associated with different life stages—puppy, adult and senior—vary depending on the size and breed of your pup. Most dogs, says Dr. Lippman, are considered puppies until they reach approximately 1 year old. But defining the transition between adult dogs and senior dogs is a little more complicated. “Large dogs tend to age more quickly than their smaller counterparts,” she says. “Very large dogs may be considered seniors at 5 or 6 years of age, whereas small dogs aren’t seniors until 10 or 12 years old.” Because of how dogs age, small dogs and large dogs also have different life spans. Smaller dogs like Chihuahuas, Yorkshire Terriers, Dachshunds and Pomeranians tend to live longer than larger dogs such as Saint Bernards, Great Danes, Newfoundlands and Irish Wolfhounds. “While we still aren’t sure exactly what accounts for this difference in maturation and aging, smaller dogs definitely live longer, on average, than very large ones,” says Dr. Lippman. “This is especially apparent when you compare very small dogs, such as a Yorkshire Terrier—which can live well into their teens—with very large dogs like Great Danes—who live to be about 10.”