While comparing human and dog ages is imperfect, there are general guidelines. The AVMA suggests the first year for a medium size dog roughly equals 15 human years. Their second year is like 9 years for us. After that, each dog year is about 5 human years.
However, dogs age differently than humans. They develop faster in their first two years but then age at a slower rate. Plus, lifespan varies widely between breeds.
A more useful comparison looks at equivalent life stages:
A 2-3-year-old dog is entering adulthood, like a 15-21-year-old human.
A 4-6-year-old dog is in their prime, similar to us from 25-35.
An 8-10-year-old dog is middle-aged, akin to humans at 50-60.
An 11-13 year dog is a senior, like humans 65-75.