My husband had an awful fear of death and dying - but he had an even worse fear of all things medical, and being hooked up to machines, and given drugs and doctors etc. And as extreme paranoia was a very big feature of his illness from quite early on, there was no way a DNR could be discussed with him. So it was just shelved, although I knew he would not want to be resuscitated if it came to it. He had never been in hospital so it had never arisen. Until his final illness, just 9 or ten weeks before his death. At that point, the doctors in the hospital (who had already had to make the decision to resuscitate him as I couldn't get to the hospital in time) discussed it with me, and explained the implications. They told me that although his heart was strong, he had suffered respiratory failure, and they needed to know, for the future, should he be brought in again in a similar condition, did I want him to be revived again? Once I said no, that at this stage of his illness (he was in the end stages of dementia), his quality of life was very poor, he was getting repeated chest infections, his swallow reflex was gone - and I felt that by now, intervention was just prolonging his death, and I would be doing it for me not for him. Then the doctor said that actually, while they have to discuss the options, his medical opinion would be the same - it would not be in William's best interests to revive him again. So he put a DNR on the file. And at that point - when he had reached that stage of his illness - the nursing home had the form put on his file there and signed by his GP, saying that he was to be kept comfortable in the home and let nature take its course, not to be transferred to hospital again.
So, it may be a question of how fit the person is both mentally and medically? I don't know.