What makes a young player good apart from obvious role stats?

Sorry if my question is a bit rough, I've been playing FM for quite some time, trying to get better by experimenting with tactics and opening my eye for young players, and it's the latter I have problems with.

Nowadays I can grasp how to value a player's stats, determination, personality and stars, but I'm finding difficulty in some cases regarding young players, no matter the position, and I think it's more about training.

My question as a whole is: How do you judge a young player overall? What stats can you actually train on a player and how can you guide them towards what you need?

To sum it up on fewer words, what stats can you develop on a young player in different positions, and what stats can you not? Can you teach a pacey striker with good finish to be composed and to anticipate the ball/work off the ball better? Can you do the otherwise? Can you train a good anticipation, tall and strong CB to also mark the players? Or a CM with good vision/passing but to also anticipate the ball?

Basically, what are the easiest and hardest attributes to develop on a promising youngster, and how should you train them?