If he or she does not remember everyone's name--including all those grandchildren--it does not necessarily mean they are demented. Misplacing a name comes to almost everyone as the decades roll on.
If the person suddenly starts disrobing in the living room--that may be another story.
Some signs it might be clinical dementia (not all dementias are Alzheimer's, by the way):
--Asking for the same info over and over and struggling to remember their own birthday.
--Not being able to follow a recipe--say putting in salt twice.
--Not being able to drive to the store--mystified about it...where is the store?
--Giving lots of money to people who call, telemarketers or charities.
--Suddenly fearful or suspicious.
My own mother had a dementia diagnosis--not Alzheimers. She could not remember something you told her five minutes before. She also lost a lot of inhibitions--and started swearing all the time (never when we were kids). She also gave $50,000 to a horrible cab driver who exploited her--bought him a car and a gun. We stopped it. She lived on like this for 18 years and died at 95.
It is horrible to deal with and worse to think about its happening to you. But it's life. Even at the holidays.