Some people are just innately stupid all their lives, but there have also been some studies in the past few years showing that changes in people’s brains as they age beyond about 75-years-old leads to a certain tendency towards gullability. Makes them very vulnerable to salesmen and scammers.
I frankly saw this with my own father, when he reached that age. He used to be about as sharp and as wary as they come. A very deliberative man. Extremely cautious and conservative about money and expenses, and very suspicious towards solicitors. But some guy came to his door soliciting money for veterans, without any credentials or anything, and my father wrote him a big $500 check, which was way, way out of his budget. Shortly afterward, my father sort of came to his senses, and he himself just couldn’t believe what he had done.
I saw this happen myself with an aunt who was once a businesswoman and nobody's fool. She kept close track of every penny all her life. However, she also suffered from depression sometimes. When she grew older and became widowed, suddenly she had these new, younger "friends" who always needed help; they kept coming around asking for money with all kinds of sob stories. She was writing checks to them. Completely out of character for her. Luckily a family member stepped in and put a stop to it.
These stories make you wonder about humanity - do the good outnumber the evil, or vice-versa?