A Closer Look at Starting Pitcher Aging: Strikeouts & Walks

Next I’ll take a look at strikeouts and walks per age group.  For strikeouts I’m going to examine K/9 and K%, and for walks BB/9 and BB%.  Then I’ll look at the relationship between the two using K/BB per age group.

Strikeouts

Both K/9 and K% show the same trajectories.  The peak is age 22 with a 5.98 K/9 and 15.48 K%.  From there, there is a consistent fall in both metrics until about age 32, when both bottom out for the next 4 years.  After age 36, starting pitchers start to increase their strikeouts quite rapidly to ranges seen in the age 29-30 seasons.  Again, this is most likely due to the fact that only good pitchers remain at these ages, which is confirmed by the attrition rates.  After age 35 the attrition rates pick up to 42% for the next 5 seasons, so in these ages, pitchers are getting weeded out at a fairly rapid rate, leaving only good pitchers who, as we assume, are posting higher strikeout rates.  This will have to be looked at in more detail though to say for certain.

Walks

Like the strikeout data, the walk data show very similar trajectories for both BB/9 and BB% per age group.  Although, while strikeout numbers get worse for pitchers, walk numbers improve as starting pitchers age.  The graphs show that younger pitchers are much wilder than older pitchers.  Walk rates peak at the age 22, and decline thereafter until age 35, when there is a slight uptick in both BB/9 and BB%.  After age 35, the rates fluctuate for a couple years before ending on a slight decline.  What is interesting is that when the post age 35 attrition begins, the walk rates don’t significantly improve, unlike the strikeout rates.  So even though better pitchers are remaining, they aren’t any better at walking less batters.

K/BB

The K/BB looks like an inverted BB/9 curve.  Although a starting pitcher’s strikeout numbers decline over a player’s career, it never outpaces the walk rate, leading to the rise in this curve.  K/BB rises every year until age 35, where we see a couple years of up and down before beginning another rise.  It seems like for a pitcher to continue pitching in the league as he ages, he would need a K/BB of at least between 1.7 and 1.9 through his prime years.

Conclusions

As pitchers age they tend to strike out less batters, walk less batters, but produce a rising K/BB ratio.  It appears that the key for a starting pitcher to stay in the league until his mid to late thirties is to focus on pitch location.  Avoiding walks appear to be more important than  increasing strikeouts.  This makes sense too.  Strikeouts increase pitch counts, as do walks.  And while strikeouts are very good for a pitcher, learning how to pitch around hitters strengths to either strike them out, or produce weak contact is not only crucial to pitching longer into games, but also to staying in the league longer as starting pitchers age.  More work needs to be done to examine why strikeouts drastically increase after age 36.  Is it simply a function of a smaller sample of higher strikeout pitchers? Or do older pitchers strike out more batters due to a more mature approach to hitters? Later posts will hopefully examine this observation.