if it is such a large issue. He comes from a lot of money, so the money argument also does not hold a lot of water. If playing in the NBA is his goal, sure leave early if he will be a first rounder as they are the only ones insured to make a successful run on a roster. The second rounders do not fare as well.
But I say again, leaving early for Europe based on his age is not a good reason because it obviously was not an obstacle for many of his previous team mates. If he wants to leave for Europe because he just wants to do that, therein is a very good logical reason.