At around 2* CA I will start to blood them into first team fixtures - unimportant cups, 'easy win' league fixtures etc, OR at 1.5 * ish, and over 18, loaning is my go to method,. A decent club, decent facilities and AT LEAST First Team status.
I have a system (at AC MIlan and Serie A has 12 named subs so a bit easier) but 2 players to cover each position gives me a First Team squad of 22. In my u20s, the 'first choice' 11 are all at 1.5 - 2.5* CA with another 11 players covering each position. Same goes for the u18s as well.
I select 3 u20 players that are staying with me all season, they will get subs slots, come on as subs and start lesser matches and will probably get 10-15 starts per season. They may earn a full spot in the 1st 22 for the following season.
Any loan offers that come in, are considered on merit for younger players - they will get more first team action elsewhere than with me, but they need relevant first team action, so playing the same position I want them in, preferably same formation with decent training etc. and I will keep a scout watching them.
Anyone from u20s that goes off on loan, gets replaced by promoting an u18 into that positional slot. If I feel the need, I will shop for a promising u18 to replace that player as well.
So, I end up after a season with:
3x u20s who have had 10+ appearances with my first team - I now look at performances and decide whether to promote, loan out next season, sell or keep tham at the same level.
A few u20s who have had a loan spells, again, do I now keep them, use them as one of my '3' in the first team, loan them again, sell them.
If you are at Man utd, then you have u23s rather then u20s but the principle is the same - youngsters at 1-2 stars, and over 18 will benefit from frst team experince more than training only. So keep your u18s at home, training and maybe playing in he u23 team. For you 18-23 yr olds, first team experience is more beneficial, so I would keep them there for a year, getting a bit of first team experience with you, but essentially a year playing u23 football. then look at loaning for two seasons which should give you a 21 year old with enough CA to become 2nd choice for a first team position - if on a Rotation contract by now, he can cover a Key Player in terms of game time.
If after two years loans and at 20-21, it's a good time to sell if you don;t think he will make the grade for you, or even loan another season or two if you think he may develop later.
Don't become attached to them lol, more often than not, my youth can do more for the club in terms of transfer value than they will on the pitch. I bought a 16 yr old four seasons ago for 1.5 mil, he looked the mutt's nuts, now he is nearly 21 and his PA is down to only 3.5 stars, CA 2.5. Has developed well enough, but overall the club has improved and he is only looking at being a backup/2nd choice rotation player for me now. Currently good enough for a few appearances but tewo better players on long contracts ahead of him in his position. Could get 8-10 million for him if I play it right, which is another 3 16 year old prodigies into the system, or about a quarter of new defender for the first team.