Summarize this article in one sentence.
There are a variety of opinions and standards as to when dogs should be retired from breeding. However, you should follow standards you are most comfortable with to protect the health of your dog.  Many kennel clubs require a dog to be 8 years or younger to register litters. Many vets suggest that dogs of normal health should be retired from breeding around 8 years old. The strictest standard to retire a dog from breeding is 5 years of age. Consult your vet when considering breeding a dog older than 5 years old. The age of your dog needs to be evaluated with other factors including size and breed. Certain breeds of dogs should be retired from breeding earlier than others. This is because some breeds have physiological issues and other related problems that could cause complications with pregnancy.  Chihuahuas and other toy dogs should be retired from breeding around 5 years old. Large breed dogs like standard poodles should be retired from breeding around 5 or 6 years old. Medium sized dogs may be bred longer than small or large breed dogs, depending on specific medical conditions and the opinion of your veterinarian. Many responsible breeders, vets, and kennel clubs suggest that people stop breeding dogs after a certain number of pregnancies.  Consider:  Some kennel clubs will stop registering litters after a dog has whelped 4 or 6 litters. Many veterinarians advise to retire a dog from breeding after 4 litters. The higher number of pregnancies, the lower the genetic diversity within a certain breed. Many irresponsible breeders, known as puppy mills, produce large numbers of dogs regardless of the health and well-being of the dogs themselves.

Summary:
Factor in the age of the dog. Consider the breed of the dog. Think about the number of litters the dog has produced.