Conventional wisdom says that one dog year equals seven human years, so multiplying one's age by seven provides one's age in dog years. That ratio is regarded by many experts as a simplification, providing only a very rough estimate of a dog's relative age.
According to WebMD, a dog matures to a relative age of 15 in the first year of his life. By the time he reaches his fifth year, his relative age is 36.
Size is a factor in determining a dog's relative age. After the fifth year, the relative age difference between large and small breeds diverges markedly. While a 10-year-old toy poodle is considered middle-aged, a 10-year-old St. Bernard is considered very elderly.