There's no exact figure, but a knight or man-at-arms might serve an active battlefield role when he was in his early fifties, assuming he was still in fairly good health. It was common for English soldiers of the day to serve for several decades. Men-at-arms and archers at that age would likely be retinue captains or command a small company. Long-serving soldiers like that were vital for the success of the English army in the Hundred Years War. Of course, royalty and important magnates might still command troops even if they were too old or weak to really take part in the fighting. King John of Bohemia, who was 50 years old and blind, essentially committed suicide at the Battle of Crecy by charging into the English lines.