## The Stolen Base Aging Curve Depicted

Over the last two years, my fellow FanGraphs authors have discussed the effects of aging on player performance and shared various pretty curves. Eno Sarris introduced some hitter aging curves for both batted ball type distribution and isolated power. Then Bill Petti looked at the various plate discipline metrics. The one skill missing thus far has been speed.

There has yet to be a perfect metric we could look at to determine who is truly the fastest runner in baseball. There are many statistics we could reference that serve as a good proxy though, such as stolen base attempts, Spd score, triples totals, and infield and bunt hits. Up to this point, aging curves have focused on skills and ratios. But aside from Spd score, all the other indicators of speed are results oriented. While it is preferable to look at a more skill-driven metric, a stolen base aging curve still provides meaningful information.

Thanks to the mathematical wizardry of Jeff Zimmerman, presented below is the stolen base aging curve from 2002 to 2013 per 600 plate appearances.

The graph includes curves for two groups of players. The blue line with a flatter slope represents the entire population that was analyzed. The steeper orange line is composed of every player who stole at least 20 bases in any season during the time period examined.

The obvious explanation for why the All group line is flatter is because its stolen base total begins at a lower base. So the decline is going to be muted since it doesn’t have as far to fall. On the other hand, the average from the 20 steals group is higher, so when the speedsters start losing their ability to swipe bags, their stolen base total goes into free fall. While the slope is certainly meaningful, perhaps more important is the age of decline.

We have always known that speed is a skill of the young and the curves validate that this is indeed true. Whichever curve you choose to look at, a player’s stolen base peak comes during the same age range — 25 to 26. For those who were never true burners to begin with, the decline from there is gradual, eventually resulting in about a seven steal loss by age 38. For the speed demons, the slow down occurs much more quickly, with the same seven steal loss happening around age 30. By age 38, the group lost 16 steals off their peak.

So from a fantasy standpoint, the obvious conclusion is to target stolen base threats who will be between ages 25 and 26 in 2014 and perhaps reduce your stolen base expectations for players the further they are away from that sweet spot. The following fantasy relevant stolen base threats will fit into that age range next season:

Of course, each player needs to be analyzed on his own merits, but just like the magical age 27 breakout theory group unveiled on fantasy sites each year, this could be your group of stolen base breakouts. Or at the very least in the case of the three names on top, they will be able to sustain their high totals of 2013.

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Mike Podhorzer is the 2015 Fantasy Sports Writers Association Baseball Writer of the Year. He produces player projections using his own forecasting system and is the author of the eBook Projecting X 2.0: How to Forecast Baseball Player Performance, which teaches you how to project players yourself. His projections helped him win the inaugural 2013 Tout Wars mixed draft league. Follow Mike on Twitter @MikePodhorzer and contact him via email.

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Jeff

Very useful. Seems logical that the body’s aging and mounting injuries would prevent people from attempting to steal, even when success rates have been pretty good. In particular player cases, we of course have to factor in managers, too–which is why I feel particularly cursed by figuring I’d get 15+ steals from Heyward and J. Upton this year.