by Eno Sarris //

Age matters. We know, for example, that the age of a prospect matters. If he’s a 23-year-old slugger beating up on 19-year-olds in A-ball, that should be noted when looking at his stats. We also know that a slump at 37 is not the same as a slump at 27. But does the age at which a player debuts in the league matter?

It seems that it does. Baseball analyst /economist J.C. Bradbury took a look at age and career lengths (pay link) and Colin Wyers provided some summary and additional work at The Hardball Times late last year. It looks like the later you join the major leagues, the shorter your career is. Intuitively, it makes sense. If a player has to be closer to his peak to even make it to the bigs, then he will be gone earlier once he falls from that peak. To say the same thing in numbers, look at the table to the right. It shows the average career length by age of major league debut.

It’s always nice to see a player finally make it after struggling in the minor leagues for years. Ryan Ludwick, who crossed 300 major league plate appearances at 28, and Nelson Cruz (who did so at 26) are relative success stories for this group.

There’s also Garrett Jones knocking on the door after his great 2009 season. In just 358 plate appearances, Jones made an impression, especially as a slugger. Take a look at the Bloomberg Sports Fantasy Tools spider graphs on the left for an idea of how good he was last year.

While Ludwick and Cruz have gone through slumps and have made it through to the other side, Jones is currently slumping. Since he debuted at 26 and didn’t get a regular gig until last year at 28, it’s possible that we should be more worried about his current slump than we should about your garden-variety slump. After all, the average player that debuted at 26 only lasted three years in the bigs.

In Jones’ defense, players with lesser debuts might be happy to be putting up the .259/.352/.411 line that he currently sports. Those numbers are above-average compared to the average hitter, but slightly below average for a corner outfielder. But after showing a .274 ISO (isolated slugging, i.e. slugging average minus batting average) last year, his slugging has fallen off, and his current .152 ISO is below his .192 minor league ISO. Since ISO takes 500 plate appearances to stabilize, his career major-league .219 ISO is just barely statistically significant. If he did experience a little power surge, it would take Jones away from a guy on pace for 20 home runs to one closer to being useful in mixed fantasy leagues.

But it’s no sure thing that a surge is coming. For one, Jones is chopping the ball into the ground (47.7% this year, 41.8% career) while his flyball rate is low for a supposed power hitter (33.6%). Via minorleaguesplits.com, we can see that Jones has hit more groundballs than flyballs every year since 2006 in the minor leagues. Even during last year’s big league breakout, he hit just 41.3% flyballs. He’s no flyball-heavy slugger.

Most of his swing rates are similar this year to last year’s breakout numbers, but there’s evidence that the league has adjusted to Jones in the pitching mix he’s seeing. FanGraphs tracks Linear Weights, which use game-state statistics to attempt to give value to specific pitches. Jones was very good against fastballs last year – his plus-14.6 runs put him just short of established sluggers Lance Berkman (+15 runs) and Brad Hawpe (+16.4 runs) against the heater. So this year, pitchers are throwing the fastball less often: 49% of the time, versus 54.9% of the time last year. It’s up to Jones to adjust now.

Given the fact that Jones hits a lot of groundballs and is seeing a different pitching mix this year, we probably shouldn’t expect a repeat of last year’s power binge — and he probably isn’t a good buy-low candidate. Placing him in the context of other late-blooming players like Cruz and Ludwick only makes the picture worse. If you own Jones in a deep league, you might as well just hold. In a shallow league, you might find something better on the waiver wire.

For more on Garrett Jones and other late-blooming baseballers, check out Bloomberg Sports’ Fantasy Tools.