Infield Defensive Age Curves
A major consideration for any team when it comes to offering a player an extension or signing a free agent is often boiled down to his offensive production over the length of the new contract. Another consideration that is often overlooked is where the bat will play. For instance, signing a 28 year old Miguel Tejada to a 6 year contract to play shortstop is actually a very smart move. His bat played well over expectations for a SS and would become league average by about his final year of the deal. Likewise, Tejada's defense was astounding and very much under-rated. A typical age progression would have showed him to be an average to slightly below average defensive SS in his final year.
In hindsight, we now know he was two years older than admitted. This shifts the projection in the wrong way. His 5th year is now as an average offensive SS and an average defensive SS (very good hands, but no range outside of his zone). Next year should result in a precipitous decline in defense, but you cannot shift him to third base or first base. Why? You can easily find bats that provide more offensive output than Tejada for the same defense. For the Orioles, thankfully they shipped him off and at least got a serviceable left fielder and some potential arms. They also helped add to Ed Wade's long list of bad, but not life threatening trades.
Here we will try to quantify typical infield age curves for fielding. When it comes to fielding there are two main considerations when it comes to generating outs: the ability to field "cleanly" and range. Fielding cleanly or fielding efficiency is a skill that maximizes when the player has had experience at the MLB level. Aging will affect efficiency, but not to a great extent. Range on the other hand is heavily affected by aging, or that is what I would assume. As a player ages, he should experience decreased ability to cover the same territory or have his reaction time slow.
How will fielding be measured?
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.
Out of Zone (OOZ) Plays will be used to 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 were considered.
The curves depicted to the right show the effect of age on fielding efficiency (orange) and range (black). Each horizontal mark represents one run for both y axes. For instance, if a player moves upward ten lines then he has prevented ten runs from scoring in comparison to the year before. Ten runs is roughly worth one win. For first basemen, each play is worth about 0.798 runs.
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. 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.
Just like the previous graph, the curves depicted to the right show the effect of age on fielding efficiency (orange) and range (black). A single play is worth about 0.754 runs for a second baseman. Fielding efficiency maxes out around age 28 or 29. Range is maximized at age 26 or 27. Ages 27 to 29 are when fielding ability is greatest for second basemen as their efficiency rises and their range has not been greatly compromised. Most second basemen fall completely apart in their early to mid thirties, so I did not have enough data to cover that time period.
Second base appears to be a position suffers a great amount of physical degradation, but also is one of the later ones in terms of reaching a high point for efficiency. It takes several years before fielding efficiency is optimized. Taking this data into consideration, defensive second basemen are hurt by free agency for the most part (or the organizations who sign them). After the renewal system and arbitration cycles take their turns, defensive minded second basemen hit free agency with their better days behind them. It is more likely that the dropoff is far more severe than depicted on these curves due to older 2B neutralizing the aging effect.
For third basemen, a single play is worth about 0.8 runs. This position has the most costly plays barely nudging out first base. It should be noted that the reason why the corners are so costly as because those fair balls slicing down the first or third base line often result in extra bases or a difficult throw to nail an advancing runner. Plays at these corner positions are about 7% more valuable than those in the interior. Balancing that is that there are far more plays in the interior than on the corners.
Of all of the positions, we have the most information for this one in terms of different ages. Third base and second base have some similarities in terms of how long players take to develop peak efficiency at these positions. Third basemen take a little bit longer as they peak around 30 or 31. The reason for both of these might be due to learning a new position. A significant number of second basemen and third basemen are often shifted off of shortstop. This switch may take years for a player to develop properly and achieve his highest level of performance.
For shortstops, a single play is worth about 0.753 runs.
Based on the age classes we have on hand (at least three data points had to be available for each age included), we were only able to include ages 22 to 32 on this graph. As opposed to the 2B curves, these have similar apexes, but differ with where they end up. Shortstop appears to take more skill and athletic ability to play effectively. This comes as no surprise. Also, range deteriorates much more quickly than efficiency, which agrees with the 2B study. Fielding lifespan is much shorter for a SS than a 2B though. Range for a SS seems to peak around 26/27 and fielding efficiency peaks 27/28.
A quick check on Google and I find that Tom Tango did something similar in February. His findings basically agree with my own even though we calculated these in different ways. His calculations predict a decline twice as rapidly as my own. For instance, we both find the same peak, but he finds a decrease of -35 plays from peak to age 32. I find it to be -18 plays. I am not sure which is more appropriate. Perhaps considering my findings and Tango's as a range would be a good idea. That range is worth about one win. Regardless, this trend seems more unmistakable.
Points to Take Home
If these curves are accurate depictions of player ability it should be noted that there are some immediate declines for all positions at the time for extensions or free agency. A typical first baseman will lose 1.3 more games in his age 32 season than his age 28 season. Third base is more resilient as only 0.3 games lost in that transition. This retention has a lot to do with the fact that players with the ability to stick at third base will stay there while those who lose range or skill will be shifted to first base. Looking at the middle infield, A second baseman and shortstop will lose 1.5 and 1.4 more games using that same comparison, respectively.
As can be seen, a major issue with free agency is that it is rarely a source of middle infielders. This is something that most teams have figured out as you will rarely see anyone sign a middle infielder for much money these days. Your typical second baseman and shortstop will be quite useful at their natural positions until they reach about age 32, then that bat that was so useful there pales with a position change.