Lyndon Johnson didn’t contest a second presidential election.  But he was expected to do so when he won the 1964 contest; when the 1968 election took place, he had recently passed his 60th birthday, and so had not yet entered old age.

Ronald Reagan was 69 in 1980, when he became the oldest person to date to assume the presidency.  That record has since been superseded by Donald Trump, who was 70 when inaugurated.

“The Gipper’s” age was a factor that year, as it was in 1984 when he won re-election.  (Remember his famous turning of the tables on the then 56 year-old Walter Mondale, the Democratic candidate: “I am not going to exploit…my opponent’s youth and inexperience”.)

Joe Biden is a horse of a different colour.  He will be 78 later this month.  Perhaps he will contest the 2024 presidential contest at the age of nearly 82, after all, having won this time round.

But if so, many people, including members of his own party and putative administration, won’t expect him to.  So his starting position as president would be unlike that of the relatively youthful Johnson and the relatively old Reagan – both of whom were expected to fight a second term.

There’s a mass of commentary about what a Biden win would mean for relations between America and Britain (they would ease in some ways and tighten in others).  And about what it might imply for our own general election of 2024 or earlier (less than you might think).

As far as we can see, however, much less is being written about expectations of a President Biden not standing again in four years time.  There is no easy way of quietening speculation about the possibility.

For if Biden says he won’t stand again, he will immediately legitimise debate about his successor as Democratic candidate next time round – and the position of Kamala Harris, who recent precedent suggests would succeed him. (But might not – for who knows?)

And if Biden says he will stand again, he will either have to do so early, which will fail to silence the chatter.  Or late – by which time it will have destabilised an orderly presidency and party unity.  Or somewhere between the two: the least bad option, but not a good one.

All this suggests if not exactly a power vacuum in the White House, then certainly a questionmark over the authority of a President Biden – and one that will surely begin to solidify early in his term.

It is not as though the path to the kind of national unity he wants is clear in any event, even if the Democrats somehow now work their way to a majority in the Senate.

As we say, Biden will be nearly 82 in 2024.  Donald Trump will be 78, as Biden is now – and he thinks of himself as, “the youngest person.  I am a young, vibrant man”.

This election has not buried Trump beneath a landslide.  He has won more votes than he did in 2016.  His Republican following won’t vanish – and nor, surely, will he.  What odds on Trump, not Biden, contesting the presidency in 2024?