Exploring position player aging curves

Believe it or not, it's hot stove season! And yet, unless you're Doug Fister, a Mariners fan, or a Mariners trade partner, nothing has happened yet.


While a slow start to the offseason is incredibly boring, it has allowed me to get working on other things. This year, I'm planning to put together a relatively complex study about how teams have viewed risk in the free agent market over the past few offseasons, including this one. As part of that project, I need to build quite a few aging curves. I need them for position players, starting pitchers, and relief pitchers. I may need them for pitchers going from starting to the bullpen or vice versa. And I don't just need them for one year, I need them for every year this decade.


Tonight, I put together the data for my position player aging curves. To qualify, a hitter must have had at least 100 plate appearances in consecutive seasons, I then took the change in that player's WAR per 600 plate appearances (WAR/600), and averaged that for every player at every age. Then I built out a curve for each year since 2011, and compared it to the most recent aging curve available at the start of that season.


Thanks to NChill17, we can see this in one neat graphic. Unfortunately, I don't really have any observations at the moment. I haven't smoothed the curves yet, and I'm looking at a limited age range. But it's still interesting to see the changes in how players have aged year-to-year, and the curves provide valuable information for evaluating potential offseason acquisitions. The important thing in the chart below is the change in WAR/600 between years rather than the value shown on the y-axis.

You probably noticed something weird about the 2014 aging curve. Compared to every other season this decade, players aged exceptionally well all the way through age 30.

And in fact, our resident pharma expert Joe Schwarz may have given us an explanation for this anomaly four years ago.


Anyways, I know this post on its own isn't particularly useful on its own. It will hopefully serve as a start to something bigger, and maybe something you can reference when deciding whether you want to take on the $295 million owed to Giancarlo Stanton through his age 37 season.