For PG, I would train on the basis of age. Training mainly depends on skills here.
For an 18 year old, it is best to train skills that take time to train which are mainly OD and JR. Then JS and passing should be trained. Finally driving and handling can be traimed when the player is about late 20-21.
I believe that the skills in order of importance, not training order go in the order:
1. Passing
2. OD
3. JS
4. Driving
5. JR
6. Handling
As driving and handling are bot very important and easy to train it is best to train them last, especially for the U21 team. Though I don't consider JR to be that important, it and OD train slowly so it is best to train them first. OD should be trained more thlhan JR. Passing and JS are extremely important primary skills so they should be trained.
After 21 the skills should be rounded up and improved in all areas especially JS, OD and passing.