Who is the youngest player to defeat a reigning World Chess Champion in a classical chess game? A simple web search doesn't seem to readily give the answer to this question. Who is the youngest player to defeat a reigning World Chess Champion in a classical chess game?
I'll start an answer just to get going as kmartin suggested. 
I'm sure it'll be superseded by other findings.

Carlsen beat Anand in Linares 2009 when he was 18 years and 3 month old.

(Other games that came to mind but narrowly miss out: Carlsen beating Topalov, 2007, Radjabov beating Kasparov, 2003)