Answer
The answer is Lennox Boyd. James, Viscount Severn is older than Lady Louise Windsor, as he was born on 17 December 2007, while Lady Louise was born on 8 November 2003. Margarita Armstrong-Jones is also older, as she was born on 14 May 2002. Therefore, Lennox Boyd is the only one younger than Lady Louise Windsor, as he was born on 22 March 2004.