It has been widely stated that one dog year equals seven human years. This is somewhat inaccurate and determining the ratio of a dog’s age relative to human years is dependent upon the size and breed of the dog.
Aging is much more rapid in the first two years of a dog’s life therefore the first year of life represents 10-15 human years. The second year is equivalent to about another 8 years in terms of physical and mental maturity.
Small dogs tend to live longer than larger dogs and they may mature more quickly in the first few years of life. A large dog may mature more slowly at first but may be considered senior at age five. Small and toy breeds don’t become “seniors” until around age 10. Medium-sized breeds are somewhere in the middle in terms of maturation and lifespan. (Wikipedia, Dogster.com)