if they are 16-18, dont just loan them everywhere. I myself keep them until 20-21(thats when they will be unhappy at lack of playing time), i let them being trained in my reserve squad.
Loaning them to clubs with low coaching and facilities will do more damage than good.
I have top training facilities, and also good tutors to enhance their mental stats and PPM.
As you are liverpool, play your youths in capital one and FA cup, and easy game in League. As your board isnt excited in these competitions i think.
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Being Tutored is one thing loaning will cancel out. Also you cant train them to your likings of PPM, i have Maupay being an ultimate poacher with all the best PPM for a poacher.
How fast? i think personality and determination have effects, i have a regen who is unambitious, he started great, but his stat is dropping for a 19 years old.
I bought a CB, 3 years ago for 5m, he was 18 and Driven personality, i played him 20 games in 1st season, almost all game in both 2nd and 3rd season. Now, he's the best CB in EPL and probably in the world, rated 30m, top teams always has interests on him, but never bid, because i give him improve contracts every year, he was just that good. 195 cm, 20 in heading, concentration and positioning, 19 in marking tackling and anticipation. And he is fast, i think he has 15 acceleration and pace. Just the best.
It depends to facilities, coaches, training, tutor, personality and playing time. So Many factors, i just love that.