I don't know about him, but we've been hearing about it from the left for over eight years, and it seems to have cleared up. Remember Trump taking, and passing the Montreal Cognitive Assessment? Of course, the left said he cheated and attacked the doctor that performed the test. You remember all that yes?
Now over at least the last six years, left has been claiming that Biden was sharp as a tack, and suffered no cognitive decline, that he was perfectly suitable for office, and in fact, was perfect suitable to run for reelection. The left scoffed at anyone that said otherwise and covered for Biden when he refused to take the Montreal Cognitive Assessment.
The left has no credibility. Everyone knows the left will lie and say anything they think will help them maintain and or regain power.
Fortunately, the left believes everyone is as stupid as the people that support them.