The fertility of male dogs varies depending on breed and size, but most male dogs begin to reach sexual maturity at six months of age. It often takes a few extra months for larger dogs to become fertile.
Once a male dog reaches full sexual maturity, usually between the ages of 12 and 18 months, they can reproduce well into old age. Male dogs are able to mate at any time with a female dog in heat. Proper care and supervision of the dog is important to ensure the health of the puppies. The American Kennel Club recommends that dogs not intended for breeding should be neutered at around 6 months of age.