There is a commonly held belief that a dog ages 7 years for each year of a human life.  That is not quite accurate in terms of maturity.  The chart below displays an approximation of your dog’s maturity level in relation to their age.

As you can see, dogs mature quickly in their formative years and the maturation slows as they age.  It furthers the importance of their training in the early years on their behavior throughout their life.  Keep in mind that smaller breeds generally live longer than larger breeds.