Sarah Dunant argues that Joe Biden's refusal to understand his moment in history is forcing the nation to confront the fact that she is no longer young. 'In the relatively short history of America from new country to super power,' writes Sarah, 'she has always - even when she behaves badly - projected an aura of self confidence, a vitality, almost cocky certainty that we associate with youth. And for the longest time, it made for an optimism, a sense of can do, that sometimes felt like manifest destiny.' That, Sarah argues, is starting to change. Producer: Adele Armstrong Sound: Peter Bosher Production coordinator: Gemma Ashman Editor: Tom Bigwood