Any consideration of Biden's mental state should also factor in the fact that he has always been gaffe-prone (speaking without a filter) and has suffered from a stammer/stutter. This is one reason why dementia testing includes a written portion instead of simply relying on speech evidence. These types of speech impediments can often lead to moments of halting speech that unavoidably lead to the impression that the speaker is mentally deficient. There is also the excess and aged skin that he has around his eyes and the squinting appearance this leads to. Added together, it's tempting to form a narrative that he must be senile. However, senility rarely takes a vacation, and I've seen enough press conferences and speeches where Biden is forceful, direct and focused to leave little doubt that he's not senile. He simply looks like a 79 year old man who is accepting and feeling the weight of the office, and during the particularly difficult contemporary conditions of pandemic and national division. The weight of the office has in modern times had a tendency to make ordinarily intelligent people look mentally weak, e.g., G. W. Bush and Bill Clinton, although Obama suffered this to a lesser degree, I'd say.

As an aside on speech impediments, when I was in my late teens I worked retail with another teen who so profoundly stuttered that he could barely get any words out. I knew that he went to my high school but hardly saw him because he was in the special education program. If you took the time to let him get his words out, you could see that he was at least of average intelligence, however, I can imagine the type of education he got from high school was much different than the average student. Had he been properly treated by a speech pathologist in elementary school, perhaps he could have led a different life. He could only work as a trash collector, roving between all the dept store register trash bins. That was 30 years ago and there are times when I wonder what happened to him.