Why was Twenty-one chosen as the Age of Majority?

Of course the age is arbitrary - Now that some states have lowered the drinking age to 18. ("If I'm old enough to fight in a war, I'm old enough to vote and drink myself silly.") One expert put the blame on the British, saying; "Twenty-one is the approximate age when both young men and women complete their full physical growth. In medieval times in western Europe, young men of noble and knightly families normally left their homes to enter into service in the household of someone of equal or higher rank around the age of nine or so. Until fourteen, they served as pages, mostly under the supervision of the ladies of the household. From fourteen to twenty-one, they served as squires for adult knights who trained them in the knightly arts. By the thirteenth century, the apprenticeship period was seven years, until the age of twenty-one, when a young man was knighted.
http://en.wikipedia.org/wiki/adolescent_psychology