Dogs do most of their growing up in the first two years. A one-year-old dog is roughly a teenager, not a seven-year-old child, which is why the old rule of multiplying by seven falls apart at both ends of a dog's life.
The size-adjusted method counts the first year as about 15 human years, the second as a further nine, and then adds four to seven human years for every dog year after that, depending on how big your dog is. Bigger dogs age faster: a seven-year-old Great Dane is closer to old age than a seven-year-old Jack Russell.
The epigenetic clock
In 2020, researchers at the University of California San Diego compared chemical changes (methylation) in dog and human DNA, and produced the formula: human age = 16 × ln(dog age) + 31. It matches human life stages surprisingly well, but it was built mainly from Labrador Retrievers and only applies from one year of age.
What to do with the number
Use it as a prompt rather than a prediction. Once your dog is in the senior band, twice-yearly vet checks, blood tests and a closer eye on weight, teeth and mobility pick up problems early, when they are cheaper and easier to treat.