Factors impacting the correlation between the age of a dog and a human:
(1) breed of the dog (and race of the human)
(2) weight of the dog
(3) lifespan of humans and the dog breed
(4) milestones in development (sexual maturity, etc)
Using 1 dog year as equivalent to 7 human years is inaccurate.
A dog at 2 years of age is approximately equivalent to a human adult of 22-24 years. After 2 years of age the correlation between the dog and human age is roughly linear but breed dependent. Before 2 years the relationship is exponential; a dog 1 year of age is equivalent to an early adolescent.
Using simple proportions:
human age equivalent in years =
= (dog age in years) / (expected lifespan for the dog in years) * (human life expectancy)
Using the date at www.dogbreedinfo.com for dogs from 2 to 18 years:
human age in years =
= (4 * (dog age in years)) + 16
After 18 years the dog is older than dirt.
