Life expectancy is increasing all over the world. As a result, so are retirement ages. The striking exception: the highest level of British politics.

In the 1800s, most prime ministers were old or exhausted — or both — when they left office. That was still the norm for much of last century. But in recent decades we have had a succession of leaders who either were or imagined themselves in the prime of life when the removal vans arrived. In several cases, they were also in the prime of ambition and anything but reconciled to exchanging the adrenalin of adversarial politics for the ermined world of elder statesmanship. Now we have David Cameron, the youngest premier since Lord Liverpool, the youngest former prime minister since Lord Rosebery. He could well have another quarter-century of active life. So how should he spend it?

He will start by learning the obvious lessons from his predecessors’ behaviour. Poor Ted Heath spent 20 years undermining his own standing. Especially in contrast with Harold Wilson’s meretriciousness, there was a pro-Heath case to be made. For all his faults and misjudgments, this was a man of moral seriousness who strove to do the best for his country. He then became the incredible sulk, behaving with as much dignity as an overtired four-year-old.