It should start when players turn 33 yo. Have never seen a case where a 32 yo drops in skill.
I have a 32 year old guy who has dropped his DMI @ 9 GS by rate faster than younger teammates and faster than younger players with comparable salaries. It makes me think that small decreases in skills might start around 32, but that most often they aren't visible until 33. Likely the speed of skill drops increases with age (similar to the case where training speeds increase as guys get younger).