The first US presidential debate is over, but questions about Joe Biden's age and his fitness to stand office just won't go away. The President's performance against Donald Trump was stumbling - sometimes even incoherent - which could spell disaster for the Democrats. But what do we really know about Joe Biden's brain - and also about the health of Republican candidate Donald Trump? This bonus episode of The Global Story was published in November 2023, but it's still relevant. It explores whether Biden really is too old to be President of the United States and the science and politics of the campaign for the White House. In the episode, introduced by Caitróna Perry, Katya Adler talks to our Medical Editor, Fergus Walsh, and our Special Correspondent in Washington, Katty Kay. TGS is part of the BBC News Podcasts family. The team that makes The Global Story also makes several other podcasts, such as Americast and Ukrainecast, which cover US news and the war in Ukraine. If you enjoy The Global Story, you may well like some of our other pods, too. To find them, simply search on your favourite podcast app. This extra episode of The Global Story was made by Neal Razzell, Alice Aylett Roberts and Emilia Jansson. The technical producers were Matt Dean, Tom Brignell and Mike Regaard. The assistant editor is Sergi Forcada Freixas and the senior news editor is Sam Bonham.