My Father began to decline around age 88. He was aware of it but tried to hide it.
Oddly, he would remember things from WWII but forget that he was in a nursing home.
Very typical that long term memory is the most resistant to decline. Short term memory is the first to go. That’s why they used to call senility”second childhood.” People could remember their birthday party when they were five but not what they had for breakfast that morning. Biden shows something different, called confabulation. When there is a large gap in memory, the patient fills it in with some story that he thinks up as he goes along just to fill in the blanks.