You become the parent – and your parent becomes the child: that happens when you realize your parent's mental ability is slipping to the point that your roles have begun to change. It need not be your parent; of course, it could be a friend, sibling, spouse, or other relative.

The first hurdle is determining when that day of role reversal has come. The realization may come suddenly with obvious signs of mental deterioration; or you may experience a slowly building awareness that something is wrong. Some people rush the day. The first "senior moment" or mistake is, to some, a sign that the senior is incapable of caring for herself. Sometimes it is done maliciously -- the adult child who wants the parent out of his life or to obtain control of any assets.

Some forgetfulness occurs at any time in life – the misplaced homework of a child or the misplaced key of a young adult. Then suddenly is misplacing an item evidence of aging? Not necessarily, without a pattern of problems.

The more common issue is not recognizing that the day has come. One may be in denial simply because one doesn't want it to be true and doesn't want to face the future without that person as a fully functioning member of one's life. 

Distance can be a factor in realizing that a problem exists. A friend of mine spoke with her elderly parents often by phone and thought they sounded fine. They reassured her they WERE fine. When she finally came to visit them, she was stunned to find her normally tidy parents living in absolute squalor. They were unable to cope with daily living.

I recommend two You Tube videos to learn more about dementia. The first is about 90 seconds, the second three minutes.