Although breeds vary, the average male dog has the ability to father puppies at 6 months of age. However, the dog is not fully sexually mature until it is 12 to 15 months old. In general, small breeds mature earlier.
Male dogs have no mating season, so they are available to breed year round. As long as it is healthy, even an old dog is capable of impregnating a female. The American Kennel Club has rules that govern this process, however. For the most part, puppies must be fathered by dogs between the ages of 7 months and 12 years in order to be registered.