Why did knights train so young?
So they would be more prepared for when it was their time to actually become a knight. They would have to start off as a page for seven years, then seven years as a squire before becoming a knight. Just think, if you were a king/queen, would you rather have a knight trained 14 years to protect your kingdom, or have them trained 5 or less?
A knight's responsibilities would be to fight when the asks him to (War/Invasion), He would often train the son's of Nobles and some of the knights would train the king's son. The knights would train to be the best knight. The also look after the poor and defend the weak, women, children and orphans.