All female mammals are born with the amount of eggs they are to use in their lifetime, unlike male mammals who continuously create new sperm. When the females are in their reproductive prime they'll be using (or loosing) these fresh eggs at a higher rate than later on when they have less. The problem with age is that by the time an animal (or human) gets older so do their eggs. This means they're less likely to be able to get fertilized and more likely to produce a baby with health issues if they are. Not only this but the body itself will break down a bit. Pregnancy is a huge strain. It'll take calcium from her bones and work her immune system over time. It might be too much and her body might naturally abort. I guess it just comes down to old animals are not bred because its bad for the animal and for its babies, not necessarily because they are infertile or sterile. Case in point, although I could not find the world's oldest dog mother I did find the oldest cat mother, "Kitty, owned by George Johnstone of Staffordshire, UK, gave birth to two kittens at the ripe old age of 30. During her life, Kitty produced a total of 218 kittens."