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).
They can be inexpensive but still very useful players in their 32nd year, but realize that their TL value drops a good amount when they get to 33.