‘States when they are in difficulties or in fear yearn for the rule of the elder men,” wrote Plutarch, the first-century Greek historian and philosopher, as he pondered “whether an old man should engage in politics”. Only the old, he believed, possessed the wisdom granted by age, and the composure that came with experience. “The state which always discards the old men,” he argued, “must necessarily be filled up with young men who are thirsty for reputation and power, but do not possess a statesmanlike mind.”

What might Plutarch have made of Joe Biden’s abject performance in last month’s debate with Donald Trump and of his insistence on remaining the Democratic candidate in the presidential election in November? Plutarch recognised that old men could be enfeebled, but “the evil caused by their physical weakness”, he insisted, “is not so great as the advantage they possess in their caution and prudence”.

Whatever his thoughts might have been about Biden, Plutarch would probably have recognised aspects of the contemporary political world. It is not just that the two men running for US president are 81 and 78 years old. American legislators are greying, too. The median age in the House of Representatives is 58, and 65 in the Senate. More than a third of senators are over 70.

Nor is it just in America that the old govern. Vladimir Putin is 71, as is Xi Jinping. India’s Narendra Modi is 73; his Pakistani counterpart, Shehbaz Sharif, a year younger; and Bangladesh’s Sheikh Hasina three years older. Benjamin Netanyahu is 74, while the leader of the Palestinian Authority, Mahmoud Abbas, is 88 and Iran’s supreme leader, Ayatollah Ali Khamenei, 85. The oldest current world leader, Cameroon’s president Paul........

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