A forum-friend has shown me the figure of how fast the player (youth) can train and how his value relate to it.
For a 18yo Player
Q29: above 4M
Q34 above 5.5M
Q39 above 8M
Q44 above 11M
Q49 above 15M
Q54 above 19.5M
Q59 probably above 25M
19 yo (-20%) value
20 yo (-35%) value
21 yo (-45%) value
and -5% value for each Quality
Now in this case this 18yo player is barely a fast trainer, not yet the fastest one. Lets do some math.
at Q29 his value suppose to be 4M but his Q is only 15 so we have to minor 5% of value for each Q
therefore his value should be 4M - 5%(29-15).4M = 4M - 2.8M = 1.2M
In fact his value is 1.17M < 1.2M I will say he is good but not the best yet.
I use that useful table above to find out which NE player I should bid on TM to ensure he is fast trainer.