Baseball is Getting Younger, but Not Uniformly So

It probably does not come as a great surprise to hear that, on average, MLB players are getting younger. There are a handful of possible reasons for that trend. Significantly, younger players are paid less than veterans. On average, they play better defense than their older counterparts, and defense is increasingly quantified and emphasized in baseball today. Younger pitchers generally throw harder, a skill that corresponds to coveted traits, like high spin rates and swing-and-miss percentages.

The amount of playing time that older players have conceded to younger ones in the past couple decades has been visualized below in several figures. Players have been roughly grouped by the types of players they are: Infielders (broadly, non-outfielders), Outfielders, Relief Pitchers, and Starting Pitchers. In the case of each type, players have been split into two groups, depending upon whether they were participating in a campaign before or during/after their age-30+ season.

For batters, “playing time” is defined here as total number of plate appearances. Infielders are (perhaps overly simplistically) just players who are not outfielders, so DHs and hitting-pitchers are included. For this group, there is a stark divergence in playing time across age groups.

In the year 2000, nearly half of infielder at-bats were made by players 30 or older. That trend held relatively steady for the first decade of the 2000s, but following 2010 infielders have been increasingly sub-30. While roughly two-thirds of infielder at-bats were made by sub-30 year olds, the dichotomy between 20-something and 30-something players was even greater in the case of outfielders.

Interestingly, the gap between the two age groups actually shrank some leading up to 2006. From there though, 30+ year old outfielders have quickly become scarce. Advanced metrics might have a lot to do with the phenomenon in the outfield; as defensive metrics have grown in reliability, older players seem to have increasingly slid down the defensive spectrum.

When it comes to relief pitchers, the age trend is really just more of the same. Here, playing time is captured by innings pitched.

Put simply, fastballs are getting faster. As aforementioned, throwing hard plays into the younger cohort’s hand given that age and fastball velocity are negatively correlated. Given these phenomena, the figure above appears to simply be a part of a confluence of factors.

Unlike the three prior groups though, starting pitchers have not experienced an obvious trend in playing time over the last couple decades.

Starting pitching is an exception in more ways than one. First, there was a larger gap in the split between 30+/- starters in the year 2000 than in the case of the other three groups. Additionally, it appears as though 30+ pitchers lost ground in the 2000s only to regain it over the last 10 seasons. In 2010, 30+ year old starters hit their low water mark for percentage of innings pitched when they threw just 28.08% of all innings from starters. In 2019, that percentage climbed back to 35.79%. Anecdotally, it isn’t a stretch to accept this. Four of the six pitchers who have thrown 200+ innings at least twice in the last three years (Justin Verlander, Zack Greinke, Corey Kluber, Max Scherzer) are at least 34 years old.

Finally, below are two tables which cover the percentage of plate appearances and innings pitched, respectively, for all batters and pitchers, regardless of position.

Infielder at-bats were down 49.08% in 2019 from 2000 while outfielder at-bats were down 57.28%. In the 2020s, it is entirely conceivable that 30+ year old outfielders get fewer than 10% of all plate appearances.

While 30+ starting pitchers experienced a bit of a renaissance in the late 2010s, that jump was offset, and then some, by the rise in deep bullpens, relief-centric staffs, and the use of Openers. In just a few years, starters transitioned from throwing >2/3 of innings (67.3%) to <60% in both 2018 and 2019. It will be interesting to see how these trends fluctuate in coming years; save for the case of starting pitchers, the decline in 30+ year olds does not appear to be stabilizing anytime soon.