The system currently works like this:
At the start of your 6th, 7th, and 8th seasons, you lose 3%, 5%, and 7%, respectively, of the points on your attributes. You can play no more than eight seasons.
Your concern with players going 9 or 10 seasons making it harder for the younger players is warranted, for sure. Personally, though, I don't see it being much of a problem, because at 13% depreciation and not being able to bank points for seasons beyond 8, a player couldn't start their 9th season with any attribute being higher than 86. Because of the harsh penalty of playing beyond 8 seasons, we might get two per season who decide to extend their career.