Is peak at age 29?

J.C. Bradbury:

Though old players may not be what they once were, the evidence indicates they can still be valuable. According to my estimates, a hitter who has a .900 OPS at his peak would be expected to post around an .850 OPS at 35; a pitcher with a peak 3.5 ERA is expected to post around a 3.75 ERA at 35. Yes, age saps athletic skill, but the stock of skill being diminished is also important.

Is this true?

Let’s look at Bradbury’s original study (unless you subscribe, it costs about $30 to download). He says:

The results indicate that both hitters and pitchers peak around age 29. This is older than some estimates of peak performance, but it is in-line with the general understanding of human physiological function. … For [linear weights],which is not normalized for playing time, players peak at 29.41 years. This is similar to the peak-age estimate of 29.13 for OPS, which is normalized for playing time.

How can we test this theory easily? Well, if peak age is 29, we should see players at age 29 have a higher OPS than they did at an earlier age, right? Some may (rightly) note there is a selection affect in who gets to play in successive seasons, and due to regression to the mean we should expect to see players decline from one season to the next – players who underperform are less likely to play in a successive season than those who overperform.

So let’s take those players who were 29 in 2008 and compare them to how they did in 2006 at age 27. Thirty-five position players were 29 in 2008 (based upon age on July of that season, the same age criteria used in Bradbury’s study). Of them, 27 also played in 2006. What we see does not seem to support Bradbury’s conclusion – on average, players lost roughly .014 points of OPS from 2006 to 2008. That’s not what we should expect to see if the average peak age is in fact 29.

Something that should also be noted – 43 players were age 27 in 2006. So a little over half of those players stuck in the league to age 29. (Of course, some of those players could have missed a season due to causes like injury or a demotion and could have returned in this past season – I don’t have 2009 stats in my database yet.) We’re looking at the survivors.

But that’s just a one-year sample, right? What if we looked at all age 29 seasons from 1997 to 2008? (As for why 1997, I shall explain shortly.) We see a similar (but less pronounced) dropoff, of about .006 points of OPS.

And of course baseball isn’t all hitting. The Baseball Databank has outfield positions broken down from 1995 on (and 1995 plus two years is 1997 – see how that works?)

POS
Age 27
Age 29
C
66
58
SS
24
23
2B
86
72
3B
29
32
CF
89
80
LF
10
18
RF
7
9
1B
113
132
DH
29
29

What we see as that as players grow older, they move off the more important defensive positions (catcher, shortstop, center field) and move to the corners. Thinking of peak only in terms of hitting is not instructive if we want to figure a player’s free agent value – defensive value needs to be considered as well.

Bradbury’s model may do a good job of explaining in retrospect where peak age is for players who make it to age 35. But it does not seem to have a lot of predictive value for what will happen to a 27-year-old player going forward. Anyone looking to predict future performance should look to some other method of estimating the effects of age.

UPDATE: Here’s another way of looking at the issue. Bradbury’s study included all batters who debuted past 1920 and played 10 seasons between age 24 and 35. Now, looking at all players who debuted past 1920, 48% were out of the league before the age of 29, Bradbury’s purported peak. Does that make sense to anyone? Or flip it around – the most common debut age for a baseball player is 23, and the average player who debuts at that age will play six years, or until age 29. In point of fact, if we look at the most common debut ages:

Age
Length
Num
21
8.02
521
22
7.14
679
23
6.00
855
24
5.18
834
25
3.90
603
26
3.15
466

So, for player that debut from age 21 to 26 (that’s 75% of players), the average career length takes them through their age 29 season. Either MLB teams are all highly irrational, discarding players the second they’ve reached their peak (when given the aging curve Bradbury proposes, a typical player should still have several productive seasons post-peak) or Bradbury’s study is simply wrong.

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Comments

  1. Bradley of Cubs Stats said...

    One thing worth note as well is HOW they age. Bradbury does note, I believe, that these hitters become more patient. If they OPS drops .014, what is their OBP doing? Personally, I’d prefer if both studies utilized wOBA, in order to nullify the unequal weighting of OPS.

    Nonetheless, the defensive switches are interesting. It is hard to say automatically, though, that their value decreased through switching positions. It looks like 2B and C are the positions that lost the most players, leaving out the most demanding, non-catcher position, SS. Maybe a larger sample size in order?

  2. Will said...

    It doesn’t address the larger issue, but the fact that so many players wash out of the league before 29 (Bradbury’s peak) doesn’t really mean much. If you assume that most of these players are on the lower end of the quality scale, it could very well be that teams are no longer interested in waiting for their peak because they’ve determined they simply aren’t very good. Also, the six year threshold is important because that’s the period before which a player can become a free agent. Perhaps that has an influence as well.

  3. JC Bradbury said...

    MLB OPS
    2008: .749
    2006: .768
    Change: -.019

    Trend of offense from 1997-2008 was declining.

    My study controls for changes in run scoring over time. It also has a much larger sample.

    On the influence of sample selection see here: http://bit.ly/4sctmE

  4. joe arthur said...

    Hi Colin,

    Good point about including fielding position.

    I get slightly different results for the most common debut age (24, not 23), albeit likely using a different definition, actual age at the debut game, since Baseball Databank provides that date.

    I can’t help feeling that all these different ways to zero in on peak age aren’t really “asking the same question.” I think the severe censoring effects of salary increase with age (combined with injury risk?) and team investment in and control of young players need to be better understood.

    Along the lines of your final question, what is the most common age of last appearance in the majors (by my definition, using age at the date of their final game recorded by BBDB)? For players who debuted at 21, it is 22. For 22 year old debuters? 23. For those who debut at 23? 24. And so on, for every debut age through the mid 30s, whether I use 1920 or 1960 as a starting point. I don’t think teams’ failure to offer MLB playing time has much correlation to these players’ peak ability.  This pattern seems more likely to mean something about finance than physiology!

  5. Tom M. Tango said...

    Guy is correct that if you want a player’s 6 best years, and at the lowest price, then simply take the 6 peak years, which, according to JC’s model is 27-32. 

    Of course, if you keep your guy in the minors from age 22 to 26, he has a chance to get hurt in those years, and then you get no benefit at all for him.  JC doesn’t consider that as part of “aging” just “attrition”.

    How about we just call what we want the “conditioning” trajectory, so that it incorporates everything we want?

  6. Guy said...

    Joe:  I agree that the cost/control issues make it very hard to model the pre-peak performance curve for players.  You would probably have to include adjusted minor-league stats, as well as MLB stats, to really answer the question with any precision.  But I don’t think it poses much problem for establishing peak age or post-peak aging trends.

    Will:  You’re right that some players get a short trial in the majors, don’t play well, and don’t stick.  But these players don’t vanish—they often return to the minors, especially if they’re any good.  If Bradbury is right that players generally peak at 29-30, why don’t we see these players continue to improve in the minors and then return to the majors?  Teams would also have an incentive to keep players in the minors until age 27, on average, to get the most value possible from their 6 salary-controlled years—but that isn’t what we see at all.  And if the true peak were 29-30, we’d see a lot of 28-31 and 27-32 careers.  But what is far more typical are short careers centered around 27.

  7. Colin Wyers said...

    I reran the query on “normalized” OPS, which is OPS divided by the league average that year and multiplied by the average OPS from 1995 to 2008. Instead of finding a dropoff of .006 points, I found a dropoff of .007 points.

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