A pitcher’s age just doesn’t matter very much. They don’t predictably gain skill as they grow, as hitters do, and they don’t decline as much as they reach their 30s. If Dempster was 26 or 36, it wouldn’t matter all that much in terms of his projection.
Well, in answer to the age thing, look at 35-year-old Derek Lowe, a pitcher nobody (apparently) is concerned about in terms of age/mileage on his arm. He and Dempster have somewhat similar career profiles (and both throw a similar array of pitches): Lowe spent 2+ seasons essentially as a reliever, Dempster, about 3. Dempster has thrown 500 fewer career innings than Lowe, but started his MLB career only one season later (1998). 1400 IP isn’t a ton for 12-year veterans — oft-injured Ben Sheets, CC Sabathia, and Tim Hudson all have more innings in fewer years, and other oft-injured guys like Kerry Wood and Josh Beckett are only about 200 innings behind Dempster. So the only concern as far as injury goes for Dempster is whether he exceeded his ideal workload last year after only throwing 66 innings the year before. Given that he began his career as a starter, I don’t think it was that big a deal for him to pitch 200+ innings in ’08. If he’s even mostly-healthy in that ballpark and with that offense and defense, Dempster should be a 14-win pitcher for 4 years. Of course, it’s the Cubs….