It doesn't happen to everyone at the same rate, but people naturally lose the sharpness they had in their youth. There are two components to this I feel:
Firstly, as people grow old, they become physically impaired and fragile. They know it takes longer to recover from illness or injury, and know they can be easily struck with illness or injury more easily. So they are more easily frightened, and more willing to listen to the scare stories the scammers feed them.
Secondly, trust is the more natural emotion, and as a person gets older it taxes their mental reserves to think otherwise; taking longer to realise they're being scammed.
