Theoretically deterioration will stop after the player loses 2 complete skill balls. Based on my experience, it takes about three seasons to lose a complete ball. So, if you can delay your player from retiring until after they turn 36 then they should stop deteriorating.
Granted, deterioration does accelerate with age, so it may only be 35 when they lose the entire second ball. I have two 34 year olds who just moved into their second balls of deterioration so I do not have firm data as to when the second ball will be completely lost.