Hitter Aging Curves

How does age affect a player's skill? It an argument that's been debated for years as general managers try to figure out when a young player might bust out or an older player might turn into a bust.

First off, there's way too much math. But it's necessary if you want to get to the heart of the issue.

In a nutshell, to do accurate work on this, I needed to go through all the hitters who ever played two consecutive seasons. If a player played back-to-back seasons, the RAA values were compared. The RAA values were adjusted to the harmonic mean of that player's plate appearances.

Consider this fictional player:

Year1: RAA = 40 in 600 PA age 25
Year2: RAA = 30 in 300 PA age 26

Adjusting to harmonic mean: 2/((1/PA_y1)+(1/PA_y2)) = PA_hm
2/((1/600)+(1/300)) = 400

Adjust RAA to PA_hm: (PA_hm/PA_y1)*RAA_y1 = RAA_y1_hm
(400/600)*40 = 26.7 RAA for Year1
(400/300)*30 = 40 RAA for Year2

This player would have gained 13.3 RAR (40 RAA - 26.7 RAA) in 400 PA from ages 25 to 26. From then, I then would add all the changes in RAA and PA together and adjust the values to 600 PA to see how much a player improved as he aged.

With that, here's an aging curve of all hitters - with additional curves for players from 1950 to 2010. (I further divided between 1950 to 1980 and 1981 to the present (600 PA season).)


Two main points can be taken from the this data:

1. The age that players peak changed from 25 to 26 when comparing the two 30-year intervals.
2. Generally, the two groups declined at the same rate. Around age 33, the recent players age at a slower rate. The reason behind this difference could be many, but upgrades in modern medicine are probably the keys.

Besides the overall curve, here are the curves for each component - hitting, position adjustment and defense.

Hitting Aging Curve - Players who reach their hitting peaks later are the main reasons for the difference in peak age with the overall curve.


Fielding Aging Curve - Similar until age 32, where the most recent players age at a better rate.


Positional Aging Curve - Players in the past continued to play tougher fielding positions as they age.


There is the general background on the creation and output of a hitter's aging curve. I will now take aging curves further and create them for the different types of players:

  • Fast players -The criteria I used were hitters with 25 or more stolen bases and at least eight triples (103 total players including Hanley Ramirez, Jose Reyes and Carl Crawford).
  • Young players with old-player skills (high K, BB and HR) - These players exhibit power at an early age with high number of walks and strikeouts. The criteria I used were hitters who are 25 years old or younger with more than 20 HRs, a K/PA greater than 15% and a BB/PA greater than 5% (73 total players including Joey Votto, Evan Longoria, Troy Tulowitzki).
  • Players with no plate discipline - Walk rate less 5% and K% greater than 20% from those with more than 200 plate appearances (148 players including Carlos Gonzalez, Jeff Francoeur, Ryan Doumit).

Here are the curves for the three types of players compared to the league average aging curve:


Some points I want to note from the data:

1. Fast and young-old players have a higher peak than the rest of the population.
2. Young-old players peak at age 25, which is one year earlier than the general population.
3. Fast players age extremely well from their peaks at 26 to 31, then they lose on average fewer then five runs over five seasons. Young-old players lose more than 10 runs in the same time span.
4. Players with no plate discipline age similarly to the overall population.

If a person has an idea about future output from a certain hitter, you can get similar types of players and compare the way they age to the general population. I'll address how other types if players age at a later time.

If you have a type of hitter you want compared, I'll gladly take suggestions.

All WAR data is from Fangraphs.com

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