Here is how I develop my youth to the best of my ability, use how you wish. I am no expert, but I think this is an ideal way of developing young prospects.
*Note - Determination doesn't directly affect how well they develop, it is actually the hidden attributes - Professionalism, Ambition
For U21/Reserves (as they will be more developed as they are older)
1.First identify your best looking prospects.
2.Next, identify if they can be tutored for the best possible personality.
-If they have a good personality already, skip to step 4.
3. If they do not have a good personality, tutor them until they do.
4. (If they have good personality) Check to see if they have attributes that make them good enough to play a few first team games, even if in cup games or against the bottom of the table teams.
5. If there is no place in your team at all (for cup games/relegation team games) and they are 19 y/o or older, look to loan them out. *It is important you are picky on who you loan them out to! You don't want to loan them to a team in an unplayable/unviewable league, where the team won't generate any games! Also you don't want to send a hot prospect to a team that is not doing well, and therefore will lose a lot of games - If the team isn't doing well, your player will likely do bad too, hindering development. Finally, make sure the team you loan them out to will actually play your player in first team football! You don't want him going to a big club to sit in their reserves! (Facilities are important also!)
6. If you aren't tutoring them, can't loan them(including to feeder clubs), and can't fit them into your team, just let them play U21 games until you can do one of the above.
7. If players are rotting in your reserves and you don't see a future at your club for them, release/sell/terminate to free their wages so you can focus elsewhere.
For U18/Youth team
1. Follow Steps 1-3 for U21/Reserves
2. If they perform well in U18/Youth, promote them.
3. Training - Use specific attribute training and create a note so you can put them on a 3-6 month training schedule.
- Focus on physicals, as they are harder to train when the player is older. Put a player on a schedule for physicals then every few months mix it up and train elsewhere, but switch back to physicals sooner rather than later.
4. If doing will in Reserves, and they are about 18 y/o or greater, loan them out following the rules for the U21 players.
(I only say 18/19 or older because thats when it is when most players start to explode and develop.)
If you have any more questions, ask me and I'll do my best to help.