The minimum age at which a male dog can impregnate a female varies from 5 to 9 months. Because dogs do not reach social maturity until 2 years of age, they should not be bred before then.
While male puppies will mount other puppies when no more than a few weeks old, a puppy cannot sire a litter until he begins producing sperm. Dogs from toy breeds usually reach this milestone faster than dogs from large breeds. Once he has become fertile, a dog will remain so throughout life, although sperm quality and quantity decline in old age. It is the breeder's responsibility to screen a dog for medical and behavioral problems before using it for breeding.