The old "1 dog year equals 7 human years" rule is inaccurate. Pets age very rapidly in their first two years of life (reaching human early-adulthood), and then the aging process slows down to about 4-5 human years per calendar year thereafter.
Additionally, smaller dogs live significantly longer than larger breeds, which is why a Great Dane is considered "senior" at age 6, while a Chihuahua might not be considered "senior" until age 10.