Well Bill James has said that a Good Pitcher, and KWood is good, has only so many innings to pitch with his arm. Maybe Wood wants to pitch into his 50's.
Seriously, he is good when healthy, but it has been 3 years since he showed that he can be healthy and that was at age 26. It gets harder as you age so unless he mysteriously improves on his durability to stay in a season he will be done by 32 or 33 years of age.