I don't even think Adu was good enough for the MLS. Teen prodigies are always a problem because often:
1) They are playing against boys. I would always be hesitant of picking up a guy who is physically menacing at 16 because if he is 5'11, he probably won't be at 22.
2) Are hyped up ridiculously, getting endorsements, which leads to poor work ethic. You can be the most talented player, but if your work ethic and head aren't right, you'll flop. See Taarabt, Balotelli, Bendtner, Macheda.