I mean, he was already walking and talking in Cataclysm, apparently immediately after he hatched (Legendary dagger questline). This must have happened less than a year after he was hatched since he was still in the egg during the Badlands questchain (also in Cataclysm). I am 90% sure that Ion or some other official has admitted that "this doesn't really make sense, but we thought it would be such a great story opportunity that we asked the players to believe some titan-magic reasoning."