WAR Aging Curves

By Jeremy Greenhouse

WAR, short for Wins Above Replacement, is an all-encompassing metric of a player's value. It incorporates hitting, defense, baserunning, durability, and spits out one number. Using Sean Smith's invaluable WAR database, I studied positional player aging.

We know that speed and defense peak early and that power and walks peak late. With WAR, we can throw everything together. Overall player value was originally posited to peak between ages 28-32, but the subject has been revisited and peak age revised to somewhere around 26-30. Here's my basic aging curve.

To develop this curve, I found all examples of players playing in two consecutive seasons, excluding the first and last year's of a player's career, since those tend to be somewhat fluky. I then computed the average difference in WAR between such seasons.

While players between 30 and 35 years old are often the best in the Majors, they are likely in decline. In general, I find that players improve at a decreasing rate until they're 27 or so and then decline at an increasing rate. I'm not trying to toss my hat into the J.C. Bradbury vs. MGL debate, but I'm using that as my benchmark for further aging curves.

My intention is to find how players, given a certain set of characteristics, age as compared to others. Height and weight are fairly consistent attributes, but unfortunately, height and weight data are unreliable for baseball players. Nevertheless, it would make sense that players with different body types would age their own separate ways, so I used body mass index to differentiate between big and small players.

Bigger is better, although the aging curves move along more or less parallel lines. You might say that bigger players age less gracefully than smaller players, but that could be just because they are better and therefore have more room to collapse. Regression to the mean works more heavily on players farther from the mean.

Next, I separated players by career defensive ability, as defined by the sum of the positional and total zone components of WAR.

Bad defenders are good hitters, otherwise they wouldn't play. I would imagine that during a bad defender's peak, he is a passable fielder. But as he ages and his defense deteriorates at a pace that outstrips the offensive decline of good defenders, the good defenders become better all-around players than the bad ones.

Separating by career hitting value,

Bad hitters peak two years earlier than good hitters. My guess is that good hitters use their power, which peaks late, while bad hitters get by with their speed, which peaks early.

Bill James once submitted that "young players with old player's skills...tend to peak early and fade away earlier than other players." Old player skills consist of striking out, walking, hitting for power, and being slow. Separating players by career baserunning value yielded no trend. I also looked at strikeout and walk rates. To do so, I had to limit my sample to years after 1954.

This evidence indicates that high-strikeout players do indeed peak a year earlier than low-strikeout players, but they also have a smoother aging curve than their counterparts. If they fade away faster, it's only because they weren't as good in the first place

By walk rate,

High-walk players actually peak a year later than low-walk players, but fade faster.

There are some lessons on regression to the mean in here. Better players appear to decline quickly because there's more room for them to collapse in case of an injury. I'm not making any conclusions about aging curves for types of players with old player skills or any such subset, since the more specifically I drill down a type of player, the smaller the sample becomes. Even so, big or small, old player skills or no, the Ryan Howard contract was a mistake.