The decline in Biden's cognitive ability began only in the last few years. He has always been a man of average intelligence and average educational achievement and he has always been gaffe-prone and awkward in his interactions with women and girls.
But as this video clearly shows, he was still fairly quick thinking in 2012 and even in 2016. But by 2020 his long, slow decline towards senility was well underway. It's hard to imagine how he can make it to the next presidential election without his senility becoming a severe handicap.
Though I am not worried about how his growing senility will affect his reelection campaign. Rather I am worried about how his obviously worsening senility will adversely affect the country right now. It's a matter of grave concern. Unfortunately it's a matter that only a few commentators and journalists are willing to address.