The maximum value (100) of a coach is very rare, less than one will appear during a season.
The coaches are superstars in coaching, real gems that are worth spening a lot of time to look for.

Using a coach is very important. Without a coach to guide them the youth players will train all stats equal and not be good at any specific position.
E.g. a defender has little use for goalkeeping skills but without a coach he will be trained as much as in goalkeeping skills as in defensive skills.
The skill of the coach is of cause important but the most important thing will be to have a coach even if he is not highly skilled.

All youth players have a hidden stat called potential.
This means that some youth players will be potential stars when you sign them while others never will improve enough for you to be stars.
Only experience from using the YTF will tell the managers what to look for when trying to sign a star.

The game will produce some superstars once in a while.
These players are very rare but will have the skills to be among the best players in the game.
If you get one of these players you will be certain to get a man that will become first team player sooner than other players and after some years be the dominant player of the team even if your team is playing in 1st division.

A youth player will spend 11-15 weeks in the YTF before he is promoted to your main squad.
Players learn at a different speed and the fast learners will have high weekly updates but will not stay as long in the YTF as the slow learners, so the slow learners can catch up with the fast learners and maybe even turn out to be better than the fast learning players.