What is a baseball player’s prime age?
It seems obvious, but it’s not. Evaluators differ on the subject. Typically, you’ll hear anything from 26-28 cited for the front end of a standard player peak to 31 or 32 at the back end.
What do the data say?
The likelihood of capturing production on offense and from starting pitchers has followed a strikingly clear pattern over the last 30 years, a period that presumably brackets the height of the Performance-Enhancing Drug Era.
Since 1984, the greatest likelihood of finding a player worth 2.0 Wins Above Replacement (WAR) on offense, as defined by Baseball-Reference.com, peaks quite clearly between the ages of 26 and 28. Each of those three “seasons” accounts for roughly 10 percent of all the players who achieved 2.0 WAR of offense from 1984-2014. The ages 25-30 are bunched closely enough to that peak plateau to suggest a period in which players are most likely to perform at something close to the height of their abilities — the so-called prime years.
The performance of pre-prime players who haven’t yet turned 25 underscores the notion that a player like Xander Bogaerts, who was 21 in the 2014 season, would have been an exceedingly rare outlier to deliver significant offensive production. Since 1984, there have been just 38 instances of players delivering 2.0 WAR of offense at age 21 or younger — roughly 1 percent of all such seasons.
Meanwhile, after turning 30, players experience a clear and steady decline in the likelihood that they’ll be productive offensive contributors, with 33-year-old players delivering 2.0 WAR with less than half the frequency of players 26-29. The picture gets progressively uglier from there.
For starting pitchers, the golden years come earlier. The greatest number of starting pitchers worth 2.0 WAR or more congregates around 25 and 26, which represent more than 10 percent of the starters who have been that valuable since 1984. The performance of pitchers at age 24 and 27-29 has been close enough to that peak to suggest a view of a starting pitcher’s prime as running from the ages of 24-29.
That doesn’t rule out the idea of a pitcher in his 30s excelling. Prime years aren’t necessarily a pitcher’s or hitter’s best years. But they do represent the years in which, all things being equal, a player is most likely to make his greatest impact.
The divide between the performance of players in their mid and late 20s compared with their 30s has been stark when looking at the period of 1984-2014. But there is evidence to suggest it’s become even greater over the last decade or so.
There was a time at the end of the 1990s and into the beginning of this century when the gap for players in their prime and post-prime years appeared to be narrowing.
From 1984-2014, there were 2.4 starting pitchers between the ages of 25-30 worth 2.0 WAR or better for every starting pitcher of similar value between the ages of 31-36 (the ages during which Jon Lester will pitch under his deal with the Cubs). But between 1997-2002, that ratio was just 1.4:1.
Viewed another way: In each of those six years, the ratio of 2.0 WAR starting pitchers between 25-30 to those between 31-36 fell below 2:1. In the other 25 seasons from 1984-2014, there were just three such years.
Offensive value saw a similarly strange period. Overall, from 1984-2014, there were 2.1 players worth 2.0 WAR or more of offense between 25-30 for every one between 31-36. But from 1996-2001, that ratio fell to 1.6:1.
Again, there was a period of six straight years in which the ratio fell below 2:1; in the other 25 seasons, there were a total of four such years.
Maybe it was PEDs allowing some players to combat the aging curve in the late 1990s and early 2000s. Perhaps there was just a generation of surpassingly good older players. Perhaps something else was in play.
Regardless, it’s worth noting that a number of recent years — during a time of escalating PED penalties — have featured a striking divide based on age between both the top offensive contributors and the top starting pitchers.
Follow Alex Speier on Twitter at @alexspeier.