We talk a lot about a player's "prime years" here at CubsDen. The general idea is that a player gradually increases production through his early twenties, reaches a peak somewhere between the ages of 27 and 32, and then declines afterwards.
This trend can be seen clearly in the numbers of one of the best pure hitters of his generation, David Ortiz (click on the picture to see the full size version):
If you look at Ortiz's home run totals, the fall off after his MVP-caliber 2005 and 2006 seasons is extremely stark, dropping almost in half from his 2006 peak to his 2008 and 2009 seasons. The data suggests a similar pattern for all batters. The chart below gives the number of home runs, on average, a batter hits relative to his age 28 season.
So, on average, a batter hits 2 home runs a year less in his age 33 season than in his age 28 season. He is also likely to hit roughly 2 home runs a year less in his age 24 season as his age 28 season.
Math stuff starts here
The numbers above, and all numbers in this article, are the result of linear regression analysis run on all batters with more than 100 PAs from 1947 to 2013. All regressions include player level controls.
Ideally, I'd like to control for seasons, as well. However, there is a mathematical problem with this. Player age and season are strongly correlated. That is, a player's who is 25 in 1985 will necessarily be 30 in 1990. Add to this a general increase in home runs over time. Because of this, the regression cannot distinguish between increases in home runs over time and the increase in home runs a player experiences on his way to his prime. In the regression run with controls for seasons, the result suggests that players hit fewer home runs every season they play (i.e., fewer home runs at 19 than 18 and far fewer home at 26 than 20) and experience large increases in home runs per season -- on the order of 100 HR per player between 1947 and 2013. The results are statistically insignificant and and run contrary to logic so they were discarded.
Fortunately, given a similar distribution of players by age over time, the results should still be unbiased without controls for season.
Math stuff stops here
However, if you look at Ortiz's OPS numbers you'll see the fall-off isn't nearly as extreme. What's happening here? One thing is that doubles are increasing, suggesting that aging players have hits that used to leave the park for home runs go for doubles, instead. I'll address this possibility in a future article. However, you'll also notice that his plate appearances are decreasing over time.
It turns out, this is specific to Ortiz alone. Instead of considering raw home runs, I considered home runs per plate appearance, or home run percentage.
These numbers show a similar pattern to the raw home run numbers, a strong increase to ages 27-29 and slow decrease after. However, notice that the falloff is nearly as strong as the raw numbers. The average batter has almost a greater than 1% higher home run percentage at age 33 than 24 and the home run percentage doesn't drop below the rate of a 24 year old until the batter hits 36 years old. It is worth noting that these are the trends seen in ALL batters. David Ortiz has a much greater increase into his prime and, similarly, a larger falloff after. The next phase to this line of analysis would be to break hitters up into different groupings (e.g., power hitter, leadoff man, utility infielder) and determine if production evolves over time differently for different types of players.
The analysis suggests that the strong production that comes during a player's prime is the result of two factors: increased production from being in a player's physical prime as well as more AB's as a player in his mid 30s experiences aches and pains that require missed games or trips to the disabled list. An important take away from this is that players leaving their prime have benefits above the raw numbers because, in effect, their performance is condensed into fewer at bats. This allows a deep team to get production out of utility players while their aging, elite players are on the bench.
Filed under: Uncategorized