It's a common misconception that dogs age seven years for every year a human ages. This is based on dividing the average dog life expectancy by the average human life expectancy. The problem is that at some points in a dogs life he ages more rapidly than his human counterpart and at other times more slowly.
Dogs typically reach maturity at about 18-24 months. Therefore, it is more accurate to calculate a dog's first two years at 10.5 dog years per year. Eighteen months would be nearly 16 and 24 months would be 21. In adulthood, dogs age more slowly. On average, large dogs age more rapidly than smaller dogs. Dogs even age different rates depending on what breed they are.
For the arithmatically inclined, here are a set of equations for calculating a human equivalent for your dog's age.
These formulas are based on comparing the average life expectancy of an American (80) with the average life expectancy of a dog based on breed. They start with the assumption that a dog of 2 years (24 months) is the equivalent of a 21-year-old human.
for up to age 2: 10.5*Age
Age 2 and above: 21+ [58/(BreedLifeExpectancy-2)]*(Age-2)
Use this chart to get the BreedLifeExpectancy for the calculation above.
These formulas appear to be the same ones used by the Pedigree calculator above. These formulas are included because the Pedigree calculator does have it's limits. According to the formulas, a 14 year old German Shepherd is equivalent to a 103 year old person. While few humans live to 100 and few German Shepherds live to 14, it is possible. Pedigree's calculator choked on that one in testing.