To see what I have already done, Part 5. It has links to the earlier parts. Generally, I don't think I am finding any unusual improvement in his performance at older ages. Yes, he has had some very good seasons when he was older, but in relation to his performance at younger ages or even immediately preceding seasons, I don't think there is anything unprecedented going on. In some cases I took park effects and league averages into account. Sometimes I also relied only on pitcher determined stats like strikeouts, walks and HRs.
Here I do something similar to an earlier part. I only look at ERA relative to the league average and how that stat at an older age compares to a younger age. So I found all the pitchers who had 1500+ IP before the age of 35 who also had 500+ IP from age 35-38. Then I ranked them according to their change in ERA relative to the league average from best to worst. The number 1 guy was Randy Johnson. Under the age of 35, his ERA was 3.36 while the league average was 4.32. 3.36/4.32 = .778. Then 1/.778 = 1.29. So Johnson was 29% better than average at the younger age. From 36-39, his ERA was 2.48 while the league average was 4.42. 2.48/4.42 = .56. 1/.56 = 1.78. So he was 78% better at the older age. Since 1.78/1.29 = 1.38. we can say that Johnson improved by 38%, relative to the league average, moving from the younger age to the older age. 3 other pitchers improved by 20% or more. Clemens was actually about 7.7% worse at the older average. Of the 101 pitchers in this group, Clemens ranked 58th.
I also used an under age 36 and ages 36 to 39 comparison. This time, Dennis Martinez improved the most, by 28%. 5 other pitchers improved by 20% or more. Clemens was about 19% worse, ranking him 77th out of the 83 pitchers. I also did ages 37-40 compared to under 37. The biggest improver was Clarence Mitchell, who did 33% better. 5 other pitchers improved by 20% or more. Clemens was about 18% worse, ranking him 81st out of the 88 pitchers.
So Clemens does not appear to have found some unprecedented fountain of youth in his late 30s. I know I did not take park effects into account here or pitcher determined stats or number of IP. But Clemens is so far down on these rankings that it is very unlikely that bringing these things in would show something unusual. Clemens did have some great years after age 39, but I dealt with that in the first part of this series. I think this will be the last part of it and next weekend I will probably have something on a different topic.