Are Player Types Aging Differently Now?

At ESPN, they recently wrapped up their prospects week. I thought I’d zag from that zig, however, and instead wrote a piece for Insider about 30-year-olds and how they’ll age. Using some research from Jeff Zimmerman on aging by player type, I tried to spot some 30-year-olds who are about to go into the tank, and some that might age better than we expect.

But while working on the piece, I asked Zimmerman to update the research on player-type aging, starting in 2005. That’s the year baseball stiffened their steroid policy. Here’s something strange: in this, what we might call the “post-PED era,” it appears as though certain player types have begun aging in an entirely different way.

First, the original aging curve. It illustrates how different groups aged by overall runs production, which includes defense, baserunning, and offense. Zimmerman detailed the delta method he used for the aging, but it’s basically matched pairs with harmonic mean work included.

From the sample of all players going back to 1950, Zimm identified three specific player-type groups, as defined below.

• Fast players were those who’d recorded 25 or more stolen bases and eight triples early in their career.
• Young Old players had hit more than 20 homers, struck out more than 15% of the time, and walked more than 5% of the time at a young age.
• Players with No Plate Discipline walked less than 5% of the time and struck out more than 20% of the time.

Here’s the aging curve for those players, as it appeared in 2011:

It seems to fit with the prevailing wisdom in baseball. Fast players age well because they can add value on the basepaths and in the field. Young Old players age well because they’ll add value with power and patience even as they age. If you have no plate discipline, you don’t have walks to fall back on as your bat slows. So, that seems fine.

Now fast forward to the aging curves as run since 2005.

Hold on. This is all backwards. What’s going on with the No Plate Discipline guys? Why are they aging best now instead of worst? And what happened to the Young Old Player Skills guys? They flipped sides. Is baseball on its head?

We know baseball has changed, just from looking at the box scores and the power leaderboards every night. But we first have to take a careful look at these curves before we run screaming to the hills.

For example. The No Plate Discipline guys do age “better” at the end. But they age worse at the beginning. They basically enter the league as meh players, and continue being meh as they go on. Also, the sample is smaller. The other curves have more than 30,000 plate appearances in most of the buckets. The No Plate Discipline curve averages around 10,000 in the middle.

And lastly, if there’s a survivor bias here — and there is in these sorts of curves, it’s almost impossible to remove from the process — it could affect the No Plate Discipline group more than the other two. If you have no plate discipline, and the rest of your game begins to suffer, you lose your spot on the roster. And then your bad stats don’t pull the curve down.

So there are a lot of reasons to believe that the No Plate Discipline curve is a red herring. Teams should not be lusting after Evan Gattis because of his poor plate discipline suddenly.

But there’s still the matter of the Young Old player curve dropping to the bottom of the pack. And the caveats from above don’t apply to that group. From age 25 to 35, the buckets in that group average 27,000 plate appearances. That seems a decent sample.

And if you can take a walk, you’ll stick around a little longer. In the Young Old grouping, the bracket from 35-39 averages 22% of the plate appearances of the 25-35 group. In the No Plate Discipline grouping, the 35-39 year old bracket averages 15% of the plate appearances of the younger group. So there’s less of a survivor bias effect, maybe.

So why would today’s three-true-outcome guys be aging worse than yesterday’s? We only have speculation at this point, but we do know the player types and what has changed.

It’s tempting to point to power being down around baseball and say that we must be getting less power from older players. But qualified batters over 35 averaged a better isolated slugging percentage from 2005 to -15 (.159) than they did from 1990 to -05 (.149).

That isn’t to say that baseball isn’t skewing younger. Players over 33 are producing less than they ever have before by raw numbers and by the piece of the pie. Look at this graph from Ben Lindbergh:

So, despite their power and patience, older players with young old skills are producing less, and that holds with the general trend in the population. Maybe it’s a quirk of how we measure defense, since this time frame includes the birth of Ultimate Zone Rating in 2002. Or perhaps it’s the rise of defensive measurements that have better put a number on the way these players lose value as they plod their way through their 30s.

Of course, a more insidious reason is still possible. It’s possible that certain substances were helping the three-true-outcome sluggers stay on the field longer in order to take advantage of their power and patience advantage for a longer period of time. Now that they don’t have those substances, the more naturally athletic players are aging better.

But we don’t really know, for one, how many players are really using right now. And how many used in the past. And if one player type had more to gain from using than others. And what exactly the role of defensive metrics is in this change in aging curves.

It’s interesting that the Young Old player — the Three True Outcome Guy — may not be aging as well, though. That would mean something about the future of the game, how the game will look, and what players will be most valued in future free agent markets.