It's different for every player and makeup has a bit to do with it. When a player has low makeup, his ratings drop at an earlier age. However, I have seen guys with average makeup still make it to age 36 with little drop in coachable ratings so the above is just more of a general statement.
What I have also noticed is that training ratings like power, range, arm and velocity develop a lot quicker than coachable ratings like glove, arm accuracy, splits, contact, eye and pitches. So if they develop quicker, I guess it only makes sense that they drop quicker. It works the same way in real life I guess, the first thing to leave is a player's speed, power and arm.
The sad thing is that I have not noticed a trend with the exception of the makeup rating and international talent -- I have noticed that International talent seems to fall off faster than domestic talent. I am not sure if that is on purpose -- maybe the sim is trying to implement some of the "lying about age" that happens in the international free agent market???