According to the American Kennel Club, both male and female dogs are considered to be too old to breed once they reach 12 years of age. A successful breeding can occur after this age, but the standard is in place for the health of the prospective offspring.
Smaller dogs typically reach sexual maturity faster than larger dogs. Male dogs are usually fertile at 6 months old and reach full sexual maturity by the 15th month. Female dogs undergo a similar timetable for fertility, although they should not mate during their first estrus cycle. Before breeding, the AKC recommends testing both partners for brucellosis, a disease capable of causing sterility or aborted pregnancies.