Between the ages of 31 and 33, Pedro Martinez averaged 31 starts and 207 innings pitched per season, posting an ERA of 3.02 with a 9.3 K/9, 2.2 BB/9 and 0.8 HR/9 while twice finishing in the top five for the Cy Young Award. At age 34, Martinez started 23 games, logged 132 2/3 innings, and saw his ERA balloon to 4.48. His career would last just 34 more games, the final 9 of them as a 37-year-old Phillie in 2009.
None of that suggests that Roy Halladay is staring down the end of his career. Nolan Ryan logged at least 200 innings per season and posted two top 5 finishes in the Cy Young between the ages of 40 and 44. For the Phillies, though, the uncertainty is the thing. No longer can they spend the offseason counting on 220 innings and a sub-3.50 ERA out of Halladay's spot in the rotation. That's not to say that they won't get it. But all you need to do is look at the career trajectories of similar pitcher from year's past to realize that the 35-year-old righthander is now at the mercy of his physiology.
Earlier today, I used Baseball-Reference.com's indispensable play index tool to find some pitchers to compare to Halladay. I wanted to find righthanded power pitchers who had logged a similar workload, so I searched for righthanders who finished their 35-year-old season with at least 2,500 innings pitched and a K/9 average of at least 6.0. I then used the pitchers in that group who had the best ERAs (Halladay's 3.30 mark ranked 13th), excluding present day guys like Tim Hudson and guys who spent time relieving like John Smoltz.
Anyway, I wanted to look at the difference in power and workload. I used innings/season for workload and K/9 for power. Here's a look at how each guys' performance differed between his 35-36 year old seasons and the three seasons prior.
Nothing really surprising here. When you look at each guys' year-by-year, there is generally a point where his performance goes into rapid decline. For Greg Maddux, it started around age 37. Most guys were around 34-35. Nolan Ryan and Roger Clemens are the only two pitchers who really defied age.