I think defenders are the hardest to identify as future great players from a young age, as they rely on maturing and getting rid of mistakes to become great. A young attacker will show his technical level, which you rarely see improving greatly after players turn 20. Although you get "surprises" like Kane, I think there are more defenders in that group, such as Pepe, Boateng, Hummels, Chiellini, Bonnucci, Barzagli, Koscielny. Although I'm not familiar with all of their careers at a young age I don't believe I ever heard about them as wonderkids.