Yes U21 - not rocket science. There are simple training systems for every position, there is a lot of information about elastic effect, potential importance, age, height meaning for training and position, etc. Just sometimes people do not find this kind of information so U21 staff should help to train at least players with good/big potential as we know lots of good talents just get lost in future 1) Poor training 2) Too competitive Team/League 3) Unactive manager and similar reasons.