It is not often that two heads of western powers celebrate a birthday in the same week. Even more rare that both are in their eighth decade. Should we celebrate that age is clearly now no barrier to such leadership jobs? King Charles III turns 74 on Monday, and will be honoured with a 41-gun salute and a rendition of “Happy Birthday” at Buckingham Palace’s changing of the guard. But US president Joe Biden passes a more significant milestone, turning 80 on Sunday. He also has a more…