I think one possible explanation could be that catchers that are still catching in the major leagues by age 32 and up have found a way to stay in the league and stay durable. Meaning, a lot of guys that were catching in their 20's are either playing a different position now (like V-Mart, Napoli, Carlos Santana, Joe Mauer) or are out of the league because they couldn't hit or their skills declined rapidly behind the plate. Figuring out how to manage the wear and tear of the position is why you get guys that seemingly last forever like Lance Parrish, Brad Ausmus, Benito Santiago, and the rare example of a guy like Carlton Fisk who was still playing at an elite level in his 40s. If you can make it to your early to mid 30s and survive as a catcher, you've probably learned something about pain management and how to maintain your place in an MLB lineup. It's not that hanging around that long makes you bullet-proof, it just means that a player has been better than most of the guys that play that position.