The Aging Curve in Major League Baseball

Ian York has taken us through the repertoires of the starting rotation for the Boston Red Sox, shown us the importance of catcher framing, discussed Clay Buchholz’s new-old changeup, revealed the evolution of the strike zone and shown how, surprisingly, umpires are actually really good at calling balls and strikes. Here, he introduces a series that will look how the age of debut and the aging curve in major league baseball relates to performance, durability and offense.

Most baseball players reach the big leagues when they are about 24 years old. The Red Sox teams of 2014 and 2015 have some outliers: Xander Bogaerts played 18 games when he was 20, and both he and Mookie Betts essentially became regulars at the age of 21. Meanwhile, Daniel Nava did not play his first game in the majors until he was 27 years old.

The figure below shows how, since 1950, age at debut correlates with longevity in the major leagues. (All these numbers are derived from the Lahman database.) The average age at debut was 24.4 years and players lasted, on average, 6.0 seasons:

But most of those players came up for only a handful of at-bats and a cup of coffee, and never made it to their second season. Since we are interested in players (Bogaerts, Betts, and Nava) who have already made it to this level, it is more appropriate to look at players who saw significant playing time. For players who had at least one season with 200 plate appearances or more, the pattern is quite different:

These players, who are at least good enough to get a reasonable number of plate appearances, are slightly younger on average at debut (23.6 years old) than the bulk of players. They are much more likely to play multiple years in the majors, averaging 9.4 seasons at this level. (Since we are interested in how long they last in the majors, this chart eliminates active players, as we do not know how long their careers will be. The 200 plate appearance cutoff also eliminates quite a few pitchers after the introduction of the DH in 1973.)

The chart shows that on average, a player reaching MLB at a young age will have a longer-lasting career, which is not surprising. Not only are players who are brought up at a young age likely to be exceptionally good, they also obviously have a much longer window before they may experience age-related decline. Players who had more than 200 plate appearances in at least one season before they turned 22 years old lasted on average a full 14 seasons in the majors.

On the other hand, while there have been over 10,000 players in the majors since 1950, only 246 of them (2.4%) had 200 or more plate appearances before they were 22. Additionally, average age at debut has been increasing for decades, after bottoming out at just under 22 years around 1965:

In 2015, the average age at debut is higher than at any time in the past 65 years, if not more. Betts and Bogaerts really are outliers.

Nava’s debut at 27 is less unusual in itself: 1,267 players since 1950 (12.5%) played their first game in the majors at age 27 or older. Unlike the vast majority of those players, though, Nava has gone on to play for Boston for three seasons and counting:

These older players, even more than the overall group, are unlikely to stick around for more than a few games. Only 171 players (1.7%) who debuted at 27 years or older ever had more than 200 plate appearances in a season. These few players still only averaged 6.0 seasons in the majors. However, since they started out 7-8 years older than the younger group, they ended up retiring at about the same age.

As a side note: Davey Lopes is a true outlier in this category. He debuted at age 27 but only saw 49 plate appearances at that age. Yet he followed up with a 16-year major-league career, playing in four consecutive All-Star games from ages 33 to 36 and finishing at the age of 42 with Houston in 1987.

Can we make any predictions about how Betts, Bogaerts, and Nava’s offense might change in the future? In the next article, we will look at when players tend to peak offensively, and how predictive their youthful performance is.

Follow Ian on Twitter @iayork

About Ian York 208 Articles
Ian is an immunologist and virologist who lives in Atlanta with his wife and two sons. Most of his time is spent driving his kids to baseball and soccer games, during which he indoctrinates his children on the glories of Pedro Martinez, the many virtues of the Montreal Expos, and other important information.


  1. has the average number of players used in a season gone up over time? I would imagine that raises the age, as more marginal players get opportunities?

    It looks like expansion had the effect of lowering the age over the short term.

    • Well, there may be more players, but the population of the US has doubled since 1950 (and of course there are far more international players as well), so I don’t think that there are relatively more marginal players.

      I see surprisingly little impact of expansion on the age chart. Baseball had 16 teams in 1960, added 2 in 1961 and 2 more in 1962, 4 in 1969, 2 in 1977, 2 in 1993, and finally 2 more in 1998 to bring the total to 30 teams. None of those dates correspond to obvious major changes in the age trends, that I can see (though there may be short-term dips and bumps — it’s hard to tell). It’s not immediately obvious to me which way expansion should push the debut age, anyway — would the relative lack of players mean that more young players got to start, or that more aging veterans got an extra chance, or both?

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