To see what I have already done, click on Part 1 and Part 2. I looked at how well Clemens did from ages 40-44 compared to how he did from 35-39.So far, I don't think I have found anything so unusual that it raises suspicions.
What I do here in Part 3 is to look at all the pitchers who had 1500+ IP under age 36 and also had 500+ IP from 36-39 (83 pitchers). This is something I actually did last August in response to a question someone asked me about Randy Johnson. Then I found their ERA relative to the league average when both old and young. You can see that Dennis Martinez is first. Under 36, his career ERA was 3.82 while the league average was 3.89. He was 2% better than average. So that is why he has a 102 for under 36. From 36-39, his ERA was 3.03 while the league average was 3.97. He was 31% better than average. That is why he as a 131 for the 36-39 column. Then 131/102 = 1.284. He was 28.4% better at the older age than he was at the younger age. This was the biggest improvement.
Clemens is ranked 76th. His under 36 ratio was 146 (meaning he was 46% better than average under the age of 36. From 36-39 he has a 117 (he was 17% better than average). So 117/146 = .801. From 36-39, Clemens was 80.1% as good as he was under 36. I need to look at more things like strikeouts, walks, HRs, fielding independent ERA, use park effects, find win values over the replacement pithcer, etc., like I did in parts 1 and 2. Also, the age profile is slightly different here. Maybe I can post that this weekend.