Over at the excellent Hardball Times site, Josh Kalk (the guy who released all of the pitch f/x pitcher cards last fall) has a new article on the Preliminary Aging Curve for Fastball Speed. Comparing the pitch f/x data from 2007 and 2008, Kalk has charted the change in pitchers' fastball speed from ages 24 to 34. His preliminary results are that pitchers' fastball speed tends to peak at age 29 and then drop precipitously after that. Specifically, he found that pitchers tend to gain 1.5 MPH on their fastballs between ages 24 and 29, but the speed tends to drop almost 1 MPH at age 30 and a total of 4 MPH by age 34. Kalk addresses some potential issues with the data, but none seem too significant.
Fastball velocity is not everything, and aging curves for pitchers overall tend to show a more gradual decline after age 29. But for certain pitchers, such as those who already throw fastballs at below average speeds or those who overly rely on their fastball velocity rather than their command or secondary pitches, the loss of velocity could be very problematic.