Two answers from two kinds of science: the size chart vets have used for years, and a 2020 formula based on changes in DNA. Enter your dog's age and adult size.
Mature adult From about 3–4 years until the last quarter of the expected lifespan (about 8–10 years for medium dogs).
Size chart (medium) DNA formula
The size chart is the one the American Veterinary Medical Association (AVMA) publishes for pet owners. It counts a dog's first year as about 15 human years and the second as about 9 more. After that it adds 4 human years for each birthday in small dogs and up to 7 or 8 in giant breeds, because bigger dogs age faster and usually live shorter lives.
The DNA formula comes from a 2020 study by Tina Wang and colleagues at the University of California San Diego. They compared chemical marks on DNA that build up with age (methylation) in people and in dogs, and matched them: human age ≈ 16 × ln(dog age) + 31. By this measure dogs race through their early years, so a 1-year-old is like a 31-year-old, then slow down. It was built from Labrador retrievers only, so it does not adjust for size.
Neither is exact. The chart is handy for everyday comparisons; the formula is a reminder of how quickly young dogs grow up. For care, the life stage matters most. The American Animal Hospital Association (AAHA) counts a dog as a puppy until rapid growth ends at about 6–9 months, a young adult until about 3–4 years, then a mature adult until the last quarter of the expected lifespan, when the senior years begin. The AVMA puts that at about 8–11 years for small dogs, 8–10 for medium, 8–9 for large and 6–7 for giant breeds.
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Try WebTraceCheckNo. The AVMA says plainly that dogs do not age seven human years for each dog year. Most of the growing up happens early: by the size chart a 1-year-old dog is about 15 in human years and a 2-year-old about 24. After that the pace depends on size, with big dogs ageing faster.
Go by the weight your dog is, or is expected to be, as a healthy adult. If your dog sits between two sizes, try both: up to about age 6 the chart gives almost the same answer for small and medium dogs, and the gap only widens in later years.
The formula uses a logarithm, which climbs steeply at first and then flattens. The study found that the DNA marks in dogs change fastest in the first year or two, so a young adult dog lines up with a person in their thirties. By old age the formula gives lower numbers than the chart, because its curve has flattened.
For the life stage, yes. The size chart starts at 1 year (about 15 human years), so younger puppies show as under 15. We show the DNA formula from age 1 too: below that its curve drops so steeply that it reaches zero at about 7½ weeks, which is not a useful comparison.