I think there are obvious signs that Biden is experiencing cognitive decline.  I don't think its to the point where he should be removed from office, but I think its a valid concern heading into the next presidential election.  

Yesterday's incident with McConnel raises the same concerns about him.

I would also add Diane Feinstein to this list.

I've never thought that Hilary was physically or mentally incapable of being the President.  I think the effort to paint her that way was disingenuous.  In Biden's case, I'm not talking about him stumbling up the stairs to Air Force One or falling off his bike.  That could happen to anybody.  I'm talking about episodes in which he forgets what he's talking about mid-sentence.  Watch clips of Biden in his 40s and 50s.  He was extremely sharp.  The gaffes that he has become known for started much later in life.

As I said in my previous post, I don't think he should be removed from office.  Right now he is competent to serve, but there are signs he is declining, and he is 80 years old, so the speed of his decline could increase at anytime.  For people to be concerned about his age in the upcoming election is not  unreasonable.  

I feel the same way about McConnell.  That looked like a pre-stroke to me.

I would have happily voted for Biden if he had chose to run against Trump after Obama's second term. But the reason I voted for him in this past election was mainly because I wanted Trump out and would have voted for whoever the Democratic candidate was. I'm hoping he chooses not to run for re-election. But if he does, I'd rather have an out of his mind Biden over any GOP candidate. 

Comparing numbers is meaningless.  There are people who are older than Biden but still in full command of their faculties (Pelosi is 83, for example), and people younger than Biden who are not. So the cutoff for how old is too old varies depending on the person.  

"As there is nothing that can be 'done' for it, I can't see the good in repeating the concern about Biden's age."

The good that can be done is awakening the Democratic Party to the need for a better candidate.