22. 21 isn't too young but is at the edge of things. younger than that and you're looking at someone who hasn't really finished adolescence, much less prepared for professional school. furthermore, their ability to interact with their peers, who would be half a decade older than them on average, would cause further friction.
note that someone who applied as a 19 year old with a 3.8/177 got dinged everywhere he applied because of his age.