Can a 15 year old dog still have puppies?

This will be a judgement call you will have to make for yourself. On one hand, a 12 year old dog can still become pregnant, which presents the problem of what to do with the puppies. On the other hand, there is really no health benefit at this point - the dog's risk of mammary and uterine cancer have been set and spaying isn't likely to change this.

However, if your dog has been diagnosed with Diabetes or Cushing's disease, she should absolutely be spayed. Trying to manage either disease with the hormone fluctuations associated with cycling female is almost impossible.