Lazy repeated the number I heard when we were shopping for a facility for my in-laws -- 50% of 85 year olds suffer from some form of dementia.
I will say it is difficult to see when someone slips over the edge and married couples seem to be able to compensate well for each other when they lose capabilities. I don't think we can ever "know" when are minds are starting to go. The ability to recognize diminished capacity is probably the first thing to go.
My in-laws were living on their own and seemed to be managing their affairs adequately. Then my MIL fell and broke her hip. The doctor told us she would never leave a nursing home. She seemed to be in control of her mind but after the fall we found out she was "seeing" people and interacting with them.
Then, it became obvious my FIL wasn't right. My wife would find stacks of mail all over the house with unpaid bills. At first she would sort them out and put them in a stack for her father to pay. He never did. Finally, my wife would stand over him while she directed every check to be written. He would argue that he didn't have enough money to pay all of his bills so he'd refuse to pay some. She discovered he was eating every meal out so she started to make him "freezer meals." He would defrost them. She put them in the refrigerator with notes on cooking instructions. They would go bad. Finally, a social worker came up to her and said how tough it must be on her to have her mother in the nursing home and her father with Alzheimers'.
Denial set in for DW but eventually we had feedback from neighbors and friends about his strange behavior that caused her to take him to a neurologist. He had the mental capacity of about a 7 to 9 year old.
We ended up putting him in an assisted living facility that has an attached nursing home for his wife. He spends his days visiting her. He has deteriorated rapidly and his mental capacity will probably force him into a memory care unit within the year.
Ahhhh annuities and their salesmen. My FIL put almost all of his available cash into a crappy annuity at the age of 82. Financially, it was a stupid thing for a person on SS and a small pension to own.
Also at 82, he was sold a "new" demo Cadillac Escallade with over 40,000 miles on it $3,0000 under the sticker price. There was a salesman who knew his mark customer.
There were lots of financial problems that we discovered. Finally, DW is accepting the fact that they are as bad as they are and that they will never be normal again. Fortunately, I insisted when they were about 80 to get wills (didn't have them!) and the POA's. They argued for weeks but eventually did it. DW has their POA and controls everything.