Dogs never stop giving birth but medically, it is recommended that breeding should be avoided after 6 years of age even they are healthy. In old age, dogs can not tolerate stress due to pregnancy and complications can occur. When dogs become old, their heat cycles can be missed for few seasons but can start again after gap. Absence of heat cycle in dogs does not affect their fertility, they still have chance to become pregnant. But in my opinion, you should not breed your dog in her old age or discus with your local vet and get fitness certificate.