However, at 78, he is the oldest ever US President. Way older than Reagan who was 69 when he was elected, Trump was 70. As we know from history, Reagan was accused of having Alzheimer's disease whilst President in the 1980's - of course much research has been done to show he was not ill during his Presidency.
Trump too has faced constant attacks on his mental well-being, with the Left saying regularly he should be removed from office to do his poor mental health. Again, whenever he saw a doctor his health came back fine.
In truth, the US Presidency is a tough, 24/7 job. If you were really ill these days I don't think it could be hidden all that well nor managed. When Roosevelt was ill in the 1940's there was a big difference in the amount of media exposure and reliance on radio for messaging, with easily staged photographs too.
So back to Biden. Already throughout the campaign he has been accused of not really knowing what day it is. In his set pieces of the last few days, you can easily see someone who is very together and is delivering his lines well - of course a lot of it will have been scripted for him, if only Trump had taken the same route maybe he would still be President next year.
I do though have it on good authority, from Feb 2018, that Harris was the chosen candidate for the Dems in the US. The fear was she would as a black woman not have a fun or successful campaign against Trump - just as Hillary had not. As such, my source (a senior Democrat figure and fundraiser) said they would look to the primaries to see how she did, and it not well enough then they would go with Biden as the candidate and her as VP.
The entire point of this was to make her President. Of course one thing we do know, with Biden being 78 is that he is going to be a one-term President come what may. They key will be that the Dems will want Harris installed so that she does not have to fight another primary and also so she gets the Presidential kudos of defending the Whitehouse. With that in mind, perhaps that plan will be to giver her a year or maybe more to get the feel of being President.
So it seems unlikely Biden will see out his whole term, but he could do yet. From here, it also seems very likely that Harris will be the next President, of course, there's many a slip twixt cup and lip.