Dogs age much faster than we do. Some people believe that dogs live 7 dog years for every one year of our life.

It's actually a little more complicated than that. In general, the larger the breed, the shorter their life span is.
Most dogs are considered senior in the last 25% of their lives.

For a list the most common breeds with their life expectancies and age at which they are considered "senior", please read When is a Dog Considered to be Senior?