First you should read the guides that exist in the forum.
Every season your player will lose a star If you get to the next level, If you stay in the same level, no one will lose stars.
You will lose money because the market value of the players decreses, because of age, not because of the star, because in the level below, people still see them at 5 stars.
Between 18 and 21 20%, 22 and 25 15%, 26 and 29 10%, 30 and 31 5%, above 32 will keep the same.