How about for example a cumulative/exponential raise in base salary starting with age 33. So we force managers to retire them, but still make it a planable step.
That makes no sense at all. If you want to make old players less attractive the better way would be to raise the possibility they get injured, or cap their stamina and/or gameshape at really low levels so they cant be big contributors for their teams.
stamina is a thing you could be pretty good at older age(lot of great marathon runners are over 30), and why you shouldn't have a good feeling in age. You already have a handicap in them loosing skills, i am not so experience yet about the speed but you at least at to expect some of them.
That they are injury prone makes sense, and could be also a way to "force" the manager to retire them at some point.
But the one problem is that their weakness get their get weak, is something which many manager appreciate in getting them for scrimmages, and i believe most employes expect rising salarys in time. i believe professional athletes aren't so strict of it, when they start to decline but i believe a bit of the old salary expectation also a good salary convince you to play longer instead of saying i stop playing now my fgood times are over.
If you have a salary increase for 33 year for example like this:
sum(2^(years above 32)* 100)
You will rarely see player older then 37 since then the extra salary would be already 6.300$(i would guess that double up his salary at least), with 40 it would be nearly 50k and at least them the player got fired. And it is the same for every player, so you can plan with it.
if you introduce it with an counter which increased every off season update when the player is 33 or older, it could be also easily integrated without running those team who already have old players.