Would it be fair to say Joe Biden was only de facto US President and if not when will he be?

Actually, "defacto' refers to someone that holds a position in practice, but not in name. What you suggest is actually the OPPOSITE of Joe Biden being a Defacto President. In the scenario you suggest, Vice President Harris would be defacto President and Biden would be only a figurehead.

I think age affects people differently. It's pretty ageist to put a one-size-fits all rule on everyone. But from what I recall, the president does have to pass a cognitive health exam. 

It is a “fact” that Joe is the  POTUS. That butthurt losers refuse to accept this fact, does not actually change this fact. FYI, Joe is more stable than the last “stable genius” that regressives elected in 2016.

Biden has never been diagnosed with any mental condition.  He did have a stutter as a child, which he overcame.  He does speak carefully and quietly, which may seem unusual compared to the angry, boisterous speeches Trump gives, but I think it's great and calming to have some who does his job without a lot of fuss.

As for him tripping on the Air Force One Steps, I guess you don't know that even Trump noted how hard they are to climb.  And Gerald Ford, Barrack Obama and Mike Pence all tripped on those same steps.

Biden is 78 years old, about the same age as Keith Richards and Mick Jagger.  Nobody calls them doddering.  And it seems you're going to have a problem voting if Trump runs in 2024, because then he will be a "doddering" 78--the same age as Biden is now.

Nope only Trumpturds believe that.

It's worse than that. He is a usurper, a traitor in the most meaningful way. He is surrounded by accomplices; seditious conspirators and co-conspirators. He is furthermore a quisling. We have watched our election system fail us (never forget that the news media and tech companies tried to help the insurrection), and the hope is that God is willing to help us against them. Any checks and balances for justice have been useless, and none of this should ever happen again.

quisling

noun

a person who betrays his or her own country by aiding an invading enemy, often serving later in a puppet government; fifth columnist.

Yes, he's is on the phone constantly with instructions from Obama.

No, that is what the former president wanted to become after he lost the election. 

That’s fair to say 

"is only", maybe you should get checked for dementia LOL