When a pitcher ages, I’m pretty sure that their rotation also decreases. This would make sense from a physiological perspective, and with all the new information that’s available out there, I’m sure we’ll be able to officially confirm this in 5 – 10 years. My theory is that as the pitcher ages, he’s not able to grip the ball as tightly, and thus not able to get as much rotation on the pitch, so the sinker sinks less and becomes less effective.

With secondary pitches, the loss of fastball velocity is a much bigger issue because part of the reason the secondary pitch is good is because of the velocity separation from the fastball. Less separation = breaking pitch becomes easier to hit. Case in point #1 is Barry Zito.

In terms of length of career, my observation has been that guys with great stuff tend to last the longest (other than knuckleballers, who can last forever simply because the pitch puts less strain on a guy’s shoulders, hands and wrists than a fastball). A guy with great stuff who loses it still drops to average level, and has the opportunity to reinvent themselves as a finesse pitcher. A finesse pitcher or sinkerballer who loses their stuff generally doesn’t have the same opportunity, because they then have so little velocity that even pitches with great movement are hittable. Yes, there are exceptions (like Jamie Moyer), but they’re pretty rare.