Youth players are the single MOST IMPORTANT thing if you wanna build a club for long term success!
They are key to providing your starting 11 with great quality for cheap money and they are also key for your finances because you can sell of the players to make a great profit.
Youth players are all about rarity. The higher the rarity on a youth player is the better he will be and the more valueable he gets.
To find high rarity youth players you must expand to bigger and wider reach searches to find the best players on the globe. You can of course do the minor and cheap searches, but it is very hard to find great talent here (though possible with a lot of luck).
Tip! When you have found a great youth player, give him fantastic training so he can reach his full potential. Especially doing training camps each season is VERY important.
Youth player will typically be developed into good players around age 22-24. Age age 26-28 they will be close to their full potential and top quality.