1B Aging Curve

Last week, I posted about how age affects fielding performance for second basemen and shortstops. These are positions where teams often place some of their more athletic players. Today, we focus on first basemen, which is a position where teams do not regard athleticism much. First base is an offensive position and being adequate is often enough for the manager to keep slotting a mashing, poor fielder in at first base. A particular interesting sidelight is that Mark Teixeira is coming into free agency this year. A significant portion of Teixeira's value comes from his defensive abilities. He is a streaky hitter, but his defense is supposedly constant. Just looking at RZR for Teixeira we see this (for what it is worth .750 is about average for a first baseman):


24 .799
25 .815
26 .840
27 .752
28 .798

Teixeira is a very good defensive 1B, but the question is how do defensive abilities of 1B age?

The main fielding considerations for generating outs are the ability to field efficiently and range. Fielding cleanly or fielding efficiency is a skill that should maximize when the player accrues experience that assists in coordination, but then tapers off when age affects reaction time. This includes being able to field balls straight from the bat and the ability to catch balls thrown to first. Range typically has a shorter learning curve (i.e., first step), but more rapidly deteriorates due to age. Range typically is not well considered for 1B. Regardless, both of these general assumptions should apply to the defensive capability of firstbasemen.

Revised Zone Rating (RZR) will be used as a surrogate for fielding efficiency. This metric assumes there is a given territory that a defender should be expected to cover. Of all the balls that pass through this zone, outs are recorded and compared to the number of chances. This is not ideal as RZR will be effected eventually by decreased range, but it should be rather representative because players are typically moved off positions if they are so unable to defend this standardized area. This is also hampered by the fact that it does not consider the ability to catch errant throws. I imagine that this skill may increase at a later date and that this graph may actually shift a year back, but I could be wrong with that thought.

Out of Zone (OOZ) Plays represent range. These plays are those that are made outside of the zone designated to the position. Again, there are potential issues. If a defender is playing next to a player who has great range then the number of OOZ plays he can accrue will probably be reduced. The resulting effect may not be great because several players seasons will be used to determine the aging curve line.

The data was collected from the Hardball Times fielding statistics. Fielding performance was recorded from 2004-2007. Out of Zone plays for each player was divided by the number of innings played and normalized over 162 9 inning games. Ages were then determined and applied to the seasons. Ages with less than three data points were removed from consideration. Only full time players with more than one season at 1B during this time period were considered. Due to the lack of players at the outer age classes, this graph covers years 24 to 32 only.

Firstbase Fielding Age Curves
The curves depicted to the right show the effect of age on fielding efficiency (orange) and range (black). Each horizontal mark represents five plays for both axises. The RZR line has been normalized for the average number of chances encountered at 1B for a full 162 game 9 inning season. For instance, if a player moves from one horizontal line to the next over the course of two seasons then the player has improved or declined by five plays. A single play is worth about 0.6 runs, so each line represents 3 runs saved or lost. Three lines roughly represent a win gained or given.

Based on the age classes we have on hand (at least three data points had to be available for each age included), we see absolutely no acclimation for range from 24 onward. Range basically plateaus between age 30 and 31 seasons. This differs from 2B and SS curves that actually seemed to require an increase in experience to maximize range. Fielding efficiency follows a similar pattern to that of second basemen and shortstops. Players peak between 26 and 27 age years. This actually goes along with Teixeira's performance so far.

Conclusion
For first basemen, fielding range is maximized in the early 20s and immediately declines until about age 30. First base is not considered a defensive position, so when a 1B ages it is typically met with a shrug. Fielding efficiency maxes out at 26 or 27 years of age and then goes into decline. It could be argued that the decline is fueled largely by the decline in range. It should be noted that efficiency, for a short period, does increase as range decreases. The reduced ability to field effectively is most likely due to age and range. A simple regression found correlation between range and RZR to be an order of magnitude greater than age and RZR.

With regard to Teixeira, his better defensive days are behind him. Although, it should be noted that only once in the past five years has he been anything close to average. He is an excellent defender. I imagine, based on the curve, that he won't being average in the field until he is about 34. Whoever signs him should do quite well with him manning 1B. 20MM per year well? That is another story.