players start to drop skills before they get 30. I'm not sure if it's 28 or 29. Anyway, this effect is very small.
When they hit 30, to 32, it goes a little faster, but you still only notice if the player was low in his subskill. After that, it faster still, but I believe that until 34, it's not realy hurtfull.
BUT, even when we do not see a down-pop, they DO still lower in skills, so they do get less good each week...
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