I should have added that people with dementia can still be competent. It is not like the door on competence slams shut when someone is diagnosed. A person with mild to moderate dementia most often can make their own healthcare decisions. In this circumstance the hospital wouldn't have to call anyone before surgery.
Generally, yes. It is true for anyone. But there are special circumstances when a person's life is at risk and no one is available. A good example would be if someone is bleeding internally and will die unless something is done.
Why do you ask?