Niall Ferguson is a professor of history at Harvard University. He is also a senior research fellow at Jesus College, Oxford University, and a senior fellow at the Hoover Institution, Stanford University. His Latest book, Civilization: The West and the Rest, has just been published by Penguin Press.

Reading David Cameron’s biography prepares you for a character out of Downton Abbey. His paternal grandmother was a direct descendant of King William IV, so technically he is a fifth cousin of the queen. His maternal grandfather was a baronet. His father’s ancestral home is Blairmore House in Aberdeenshire. He was educated at Eton and Brasenose College, Oxford, where he was a member of the notoriously toffee-nosed Bullingdon Club. His father-in-law is another baronet.

Yet when you actually meet Cameron he’s anything but a throwback to the Edwardian era. Disarmingly free of snobbery, he is Dave to his inner circle, hates wearing a suit and tie, and is as happy watching 30 Rock as Downton Abbey on TV. The only clue that Cameron is to the manner born is the seemingly effortless way he shoulders the burdens of power. He must be the first prime minister in history to look younger after nearly two years in office.

Cameron is in fact five years younger than his American counterpart. But when he meets President Obama in Washington this week, the age difference will look more like 10 years. Power has visibly aged Barack Obama. It has rejuvenated Cameron…