One year of dog life is equivalent to 18 years of human life. A dog's age of 1-9 months is equivalent to a human's age of 1-13, a dog's age of 1-10 is equivalent to a human's age of 18-60, and a dog's age of 11-20 is equivalent to a human's age of 63-103. Since a dog's lifespan is usually limited to 15-20 years, we can say that a two-year-old dog is a young dog. A nine-year-old dog is a middle-aged dog, while a 14-year-old dog is an old dog.