It's a complicated issue. Here is my MIL story.
My MIL is very sharp at almost 90. She calls and says some guys stopped by, they were sealing driveways in the neighborhood and could they give her a quote. She proceeds to get the quote, which is $129 for x square feet. I tell her I think it is high, after telling her for the hundredth time she should not open her door or get quotes from strangers, but when DH gets home he will call her. I hang up the phone, and as I am thinking about it, she has nowhere near the square footage they quoted. So I call her back, and she goes out with a measuring tape and sure enough, they quoted her a square footage 2.5 times what she had.
She starts talking to neighbors, most of whom are elderly, and sure enough, they all got these inflated quotes, but no one had measured themselves. They called the police who tracked the guys down, they were not licensed to door to door solicit.
So in her case, it was not lack of cognition but being trusting and for all her years naive about modern ways of the world.
But the best you can do is keep active (she still swims a mile four or five times a week and walks the dog twice a day), keep mentally active, maintain social ties and in her case, she calls DH before doing anything. Hopefully that will continue