Trillkott

First of all, Im assuming that all AI teams only use the standard general training.

So what is the best way to make those promising youngsters develop as fast as possible?
Sending them out on loan for constant match practice, or let the rot in the reserves with a custom (and obviously "better") training schedule?

Bongo-Bongo

It really depends how good the player is.

The more the player improves, the less useful it will be for him to play in the reserves. The level of competition is minimal, so it is best to try and send your better youngsters out on loan to get first team football, otherwise you run the risk of them stagnating.