Fun With Aging Curves

It’s long been known that major league baseball players traditionally peak in their late twenties, hang on through their decline phase in their thirties, and, if they’re lucky, maintain a modicum of value into their late thirties. Various analysts have used various data samples and assumptions to plot the aging curve on a graph. The below curve was stolen directly from Jeremy Greenhouse of The Baseball Analysts, and while I won’t comment on the relative merits of this study and other, similar ones, his logic seems reasonable enough for me to use it to draw ridiculous conclusions.


I should note that I shifted every data point up by two wins to keep all the WAR numbers positive. This way, I could assign a year-to-year change percentage to every two-year group. You might call my work unscientific. I might call you a jerk.

Let’s start with our old friend Barry Bonds. Of every position player ever to play baseball, only Babe Ruth has earned more WAR (per baseball-reference) than Bonds’s 162.4, and the two are basically tied at the top (Ruth had 163). The argument that Bonds is the greatest baseball player of all time doesn’t sit well with some people because his late career surge was- shall we say- chemically enhanced. Disregarding the similar chemicals injected by pitchers of the time and the changes in ballpark dimensions and strike zones that had scores of players launching home runs at record rates, let’s use the average player’s aging curve above to take a guess as to what Barry’s late career might have looked like without pharmaceutical help.

Here’s Bonds’s actual WAR curve:


Bonds’s first peak came from ages 24 to 28, when he put up annual WAR figures of 8.0, 9.7, 7.9, 9.0, and 9.9. After the strike kept him to 6.2 and 7.5 WAR in ’94 and ’95 (his rate stats held steady, but he played fewer games), he shot back up to 9.6, 8.1, and 8.1 in his early thirties. By age 33, he had earned 99.5 WAR, more than all but 18 prior players had accumulated in their entire careers. At 34, he suffered through his first injury-plagued campaign, held to 3.8 WAR despite a 156 OPS+ over 102 games. If we assume it was after that season when he started using steroids (or at least got better at using steroids), we need to establish a baseline for his age 35 season. If we assume that injury was a harbinger of things to come, we could reduce his 3.8 WAR by the ~14% most players lose after age 34. If we assume 1999 was a fluke, we could reduce his 8.1 WAR from 1998 by the 12.5% most players lose at 34 and 14% for 35. I chose to average these two figures with his 1997 campaign, regressed three times, and arrived at an expected WAR of 4.9 in 2000. This feels conservative, as aside from ’99, 4.9 WAR would be Barry’s lowest total since his age 21 season, when he hit .223 for the 1986 Pirates.

From there, we keep slashing away at his numbers, cutting half his age 35 WAR by age 38 and all of it by 40, consistent with the experience of so many one-time superstars. This demolition of the end of his career leaves Bonds with 120 WAR, which would be good for 11th all time, between Ted Williams and Alex Rodriguez. Here’s the cynic’s view of age ravishing the greatest player of his generation:


Now we’ll jump ahead a generation, to a player putting up numbers nobody except Bonds has managed since the days of Mantle and Mays, and numbers nobody has ever achieved at his age. Mike Trout was worth 19.7 WAR over his first two full seasons (fangraphs gives him 20.4), which he played at 20 and 21. Applying the Greenhouse aging curve to Trout’s early career, we get this:


Yeah, those are four 14-win seasons. Babe Ruth had one season at exactly 14 WAR, and he’s the only player ever to top 12.5. Trout projects to have eight 12.5-win years. Again, I used a blended approach to projecting Trout’s age-22 season (this year), averaging 10% growth on his age-21 year and two years of growth on his superior age-20 season.

Total WAR? 210. That’s 29 percent more than Ruth. If we include Ruth’s 20.6 pitching WAR, standard-aging-curve Trout still beats him by 26 wins, or three full MVP seasons.

But we’re not done. Let’s say Trout has his own personal McGwire/Griffey/Sosa (we’ll call him B. Harper- no that’s too obvious- let’s call him Bryce H.) breaking home run records, winning championships, and getting more attention than Trout gets. By age 35, Trout is down to a human (though still superstar-caliber) 7.5 WAR, not far off of Bonds’s last two healthy seasons before the video game numbers shrouded by so many asterisks. Let’s send Trout to Barry’s pharmacist and apply the Bonds ending to his already-legendary career. Those four seasons Bonds had in his late thirties, which are some of the best seasons in baseball history, would actually fall short of every season in hypothetical Trout’s nine-year peak. Still, they’d give him a career that looks like this:


…and a tidy 243 WAR, roughly equal to the career totals of Barry Bonds and Jeff Bagwell. Or Mickey Mantle and Tris Speaker. Or Derek Jeter and Ernie Banks and Mark McGwire and Jim Rice.

I don’t think Mike Trout’s career will follow any aging curve we’ve ever seen, but if it follows a typical one, with or without the Bonds ending, we’re going to need some new record books.

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3 Responses to Fun With Aging Curves

  1. sophiebowns says:


  2. Barrie Pollock says:

    Fun essay, Bryan.

  3. hankgillette says:

    Interesting, and I hope to see it (the Trout natural aging curve).

    Others have suggested that players who start out at such a high level are more likely to have an Al Kaline type career (Kaline didn’t really get any better than he was at age 20, although his best WAR total was at age 26).

    Any chance you could do a Kaline style aging curve for Trout?

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