Simplify the following text for a Grade 3 native English-speaking person.
Original: He was born on 6 December 1421 at Windsor , and succeeded to the throne at the age of nine months as King of England on 31 August 1422 , when his father died , and King of France on 21 October 1422 upon his grandfather Charles VI 's death in agreement with the Treaty of Troyes in 1420 .
Simplified: He was born on 6 December 1421 at Windsor . He became king at the age of nine months on 31 August 1422 , when his father died .