Old and in the way?
As an old man is replaced in the world's most powerful political role by an even older man, why are new faces - and new ideas - finding it so hard to break through?
So a 78-year-old is taking over from a 74-year-old. It hardly feels like a leap into the future, does it?
It's unsettling that so many of those leading our world are past retirement age. They are seen as too old for a regular job - but are welcome to take on a role which demands not just wisdom and experience but also stamina, resilience, late nights, early starts, huge stress, incessant travel, next-to-no time off ... a perpetual maelstrom even in normal times, and in case you hadn't noticed, these aren't normal times.
The ageism which banishes those at the peak of their careers to the scrapheap at some arbitrary birthday is, of course, cruel and wasteful. No one would want a ban on senior citizens standing for elections and taking the top jobs. But the preponderance of leaders who are, let's say, getting on a bit does tell us something a little uncomfortable about the world's inability to change.
If we take the combined age of outgoing and incoming US presidents and then jump that far back into the past, we land almost in the era of India's Great Rebellion and the US Civil War... events which seem to us remote.
When Joe Biden was born, India was under British Imperial rule; Nehru was in jail having launched the Quit India movement; Netaji's Indian National Army was just being assembled; and the viceroy was somebody you've almost certainly never heard off, the wonderfully named Victor Hope, aka the Marquess of Linlithgow. It feels a long time ago - because it was a long time ago.
And while the United States is first among gerontocracies (nations ruled by the old), there's a fair bit of competition. Nigeria's Muhammadu Buhari is 78, Aung San Suu Kyi is 75, Bangladesh's Sheikh Hasina is 73, Hassan Rouhani in Iran is 72, Israel's prime minister Benjamin Netanyahu is 71, and of course Narendra Modi is 70. Imran Khan, South Africa's Cyril Ramaphosa and Vladimir Putin are all 68, while China's Xi Jinping is a youthful 67.
Britain's head of state is 94 and the heir to the throne 72 ... which make Boris Johnson, the prime minister, seem positively juvenile. He was, after all, born after the Beatles' first big hit. (He's 56).
Politics like most careers is usually a hard slog, serving time in all sorts of junior roles before the opportunity arises to climb the greasy pole of political power. But it's not always been like that. Rajiv Gandhi was 40 when he first became prime minister; Tony Blair was 43; Bill Clinton entered the White House at 46; Barack Obama was just a year older. Whatever your view of them, their relative youth was part of their appeal: they embodied hope, a fresh start, a new dawn.
There are still a handful of world leaders who have bucked the trends towards the grey and elderly: Canada's Justin Trudeau came to power aged 43; even more striking, the French president, Emanuel Macron was 39 when he was elected in May 2017, aided by a political landscape which isn't simply a two-party system and a form of election (with a second-round run-off between the two leading candidates) which offers more scope for political surprises.
The first-past-the-post voting system is one of the main causes of the failure of politics to renew itself. In the United States and Britain, it has propped up a stale two-party system by making it all but impossible for new nationwide parties to win any substantial number of seats in legislatures. That has proved a block on new approaches and new people getting political lift-off. Green ideas are gaining traction in both countries - but the Green parties which are their most natural champions remain on the fringes because they don't have the concentrated support base which will give them representation under the current dispensation.
The antidote to ageing men at the helm is evident in some of the smaller and more vibrant democracies: New Zealand's Jacinda Ardern, widely acclaimed for her handling of a whole range of crises and calamities, was 37 when she became prime minister; Sanna Marin in Finland was 35; Iceland's prime minister, Katrin Jakobsdottir, took office at 41; and Nicola Sturgeon became Scotland's first minister (no Scotland is not independent - yet - so her role is not quite comparable with the others) aged 44. All of these younger women leaders are regarded as having performed exceptionally in their nations' top jobs.
All the same, Joe Biden's advent to power reflects a trend towards older leaders. Whatever their merits, they are not digital natives - they don't quite 'get' the new, digital world. And they don't feel the imperative in addressing climate change that so many of the young instinctively understand. They are consolidators rather than agents of change. Sometimes stability is comforting, but a new and urgent approach to addressing environmental degradation and profound inequality might be even more reassuring.