The debate about how old is too old to serve in public office has resurfaced this week after the shock death of the Republican Lindsey Graham and the surprise return of Mitch McConnell, the 84-year-old senator who published a photo of himself in hospital after a long absence from the spotlight. With rumours continuing to swirl around Donald Trump’s health, why is it that US politicians seem to cling on to power for so long? Jonathan Freedland speaks to Alexis Coe, a presidential historian and columnist for the New York Times Book Review, about whether the US is becoming a gerontocracy