No doubt about it, hitters are getting older. A little over two months into 2007, the average age per plate appearance sits just below the all-time high achieved in 1945, when most young men were off fighting World War II. Why is age going up among hitters, and what are the implications for baseball?

Age per plate appearance measures not just the distribution of age on a roster,
but the actual use of older players by managers in a lineup. To calculate this statistic, multiply seasonal age (age before July 1) by the number of plate appearances by the player. Sum those for each player who batted in a season, and divide by the total plate appearances. The following graph shows Age per PA by year from 1901 through 2007:

Since the start of the twentieth century, baseball spent many years with an
average age for batters approximately between the 28 to 28.5-year mark. A big dip began in 1909, and reached bottom in 1912-1913. According to The New Bill James Historical Abstract, baseball was expanding the number of
players used at that time, and managers started using specialized players like
pinch hitters and relievers more. As rosters expanded, it’s possible they were
filled with younger players. Baseball attendance growth was low in this time
period, with a number of leagues folding later in the decade. In a down period, young players tend to be cheap labor, so that may be a contributing factor as well.

After that, with the exception of World War II, average age stayed in a narrow
band for about forty years. With the advent of expansion, player ages plunged in the 1960s. The 100 new jobs gave opportunities to cheaper, younger talent. The 1960s was also a low-growth decade for attendance. Baseball during this era was also at the height of control of players. The implementation of the amateur draft removed the chance to sign with a team for a bonus before entering into the reserve system. No mechanisms existed to force the salaries of older players higher. If a team decided to cut the pay of a veteran, he could sign or leave, and it looks like many decided to leave.

Age started climbing a few years after the advent of free agency. Two factors
worked to keep older players in the game at this point. Long-term contracts forced teams to get the most out of their investments; a hitter is unlikely to be benched or released if a team pays him a high salary. Then, because players can earn better salaries by offering themselves to the highest bidder, the incentive exists to stay in the game as long as possible.

(When it became clear in the 1990s that Aaron’s home run record was soft, the
argument was made by Aaron himself that players would not stick around long
enough to hit 756 homers. They made too much money and would retire early, he said. Unfortunately for his record, making too much money causes most people to stay in a job, not leave it.)

As player age continued to rise, there is a natural question of whether this was a good thing. If the average age of baseball’s hitters gets too high, there could be a falloff in talent; in general, players decline after their late twenties. However, GMs and managers are actually pretty good at weeding out the good players from the bad as they age, as you can see below.

The time period 1993 to 2006 represents an era of fairly constant offense. The
PA curve represents total plate appearances by age during this period. This line clearly shows that players in their late twenties are given the most chances to bat, confirming the idea that players peak in this time frame. The On Base plus Slugging line surprised me, however. The first half makes sense–OPS rises as players age toward their peak. What surprised me was that this level remains constant, even rising as bit, as players age into their middle thirties.

When the two curves are viewed together, the reason for the steadiness of OPS
emerges. As a population, players disappear from the game as they age. The ones
that remain are the ones that can still hit. After reaching the age of 36, this becomes a bit more problematic, but up until that age, either general managers are very good at signing older players who can still play, or managers are very good at using these players appropriately, and putting them in situations where they can succeed.

Should the aging of major league lineups be a source of concern? Over the last 14 seasons, management has done a good job of keeping the good older players and letting the poor ones go or sit on the bench. As long as that continues, the level of play should be fine. The other problem with allowing hitters to age revolves around money. Right now, cash flows into the various clubs at a nice rate, so they can afford the added salary that comes with an aging offensive corps. The only caveat is that the 2007 spike in age has come alongside a down offensive year. Perhaps management took a step too far this season. We’ll have to wait and see to find out the answer to that question.

As a postscript to last week’s
, Knowledge Problem notes that New York rescinded their
anti-scalping laws
. That certainly makes dynamic pricing of tickets easier.