I would like to know if every season start, when our players become older, do they have a reset for their experience points ? I mean that I have a level 10 coaching dep., so do my youths need again on average 15 points to get “blue numbers” ?

No they don’t. I played a player in some officials. Loaned him out this season. He increased in 1 friendly. So that meens he was almost ready to increase previous season, just needed 1 more XP point which he got in this friendly.
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Answer to your question, No…..they xp received from previous season is remembered and taken into the next season.

The only exception to that Sly, is when your player had earned all the skill points possible in the previous season, in this situation the experience points are “reset”.

Thanks to both of you ! =)