Every +1 year on a player makes him lose 10% of his total value. A 20 m player will be valued 18m next season if he doesnt gain further quality.

As for values they are indicative not on talent but of training capability. A Q54 5* 19m valued player will be a faster trainer than a q54 15m. Also alwais count in years. A 19yo q 54 18m vAlued player is numerically a faster trainer than the 18yo 19m one, since if he'd be 18 its value would be 20m.

As for performances tho, usually the youngsers takes 1-2 season before e pressing all the potential. Sometimes they just play bad, other times they ultra perform since th first day.

With a full 18-19 yo main team you can expect to suffer a lil cor half season or the whole first one, then you ll have a strong team the next going one.