Why do players become weaker after the start of every new season?

[Paivinha]

Im playing from 2 seasons and every new season even players under 25 age are getting weaker

Hello vasetowe,
The stars are quality indicators for the level at which you're playing.
When you level up, your players do not lose quality, they remain with the same characteristics of last season with respect to Attack, Defence, Physical and Mental.

I will cite a simple example.

Last season I was at level 1 where we have players with 20 to 24 quality being classified for 5 stars and some players with 26 quality sorted by 6 stars.

When we climbed level, to have finished the league between the 1st and 7th place, these natualmente players lose a star, but why?
If you level up, obviously things will be a little difficult for each and every game or situation, therefore, these players lose a star that is indicative of quality for your current level and the game tells you, "Look, this player only has 4 stars and at this level the amount of stars it has (indicating the quality of it) is not that strong! "

And so on, the stars are only indicators.
I created this table for you to have a better idea about the qualdiade indicators per level.

Note that I emphasis added quality 20 for 1,2,3,4 levels, now watch the stars above, 5,4,3,2, you realize that the quality is the same but only changes the quality indicators by level who are the stars.