Can a 5 year old dog still be spayed?

This is a judgement call that you will have to make for yourself. On one hand, a 13-year old dog can become pregnant, which presents a challenge for what to do with the puppies. On the other hand, at 13 years old there is no preventive health reason to spay the dog - her risk of mammary and uterine cancer are already fixed and not likely to change at this point.

However, if your dog has been diagnosed with Diabetes or Cushing's disease, she should absolutely be spayed immediately. The hormone fluctuations associated with a cycling female make controlling both of these diseases almost impossible without spaying her first.