As with many a key sabermetric tenet, Bill James is responsible for introducing the idea that ballplayers, or at least hitters, reach their peak value at the age of 27. In fact, it was a serendipitous 27 years ago, in the 1982 Baseball Abstract, where James first presented his research supporting this conclusion, thus contradicting the then-prevailing wisdom that ballplayers peak between the ages of 28 and 32.

While you wouldn’t know it to see the way certain front offices function when signing over-the-hill hitters, this particular bit of James’ wisdom has withstood the test of time, and has even undergone some expansion. Using more advanced valuation methods, Nate Silver has found that players peak
between the ages of 25 and 29, generally cresting at 26 and 27, with some understandable variations by positions. Speed-based players, like middle infielders and center fielders, tend toward the earlier side of that range.

Baseball fans like their rules of thumb simple, however, and focusing on hitters about to enter their age-27 seasons is particularly popular in the fantasy realm. Although this year’s “Class of 27” isn’t full of marquee names whose eye-popping numbers match those of 25-year-old Hanley Ramirez, 26-year-olds David Wright and Jose Reyes, or 29-year-old Albert Pujols, it’s worth a gander. While I’ll eschew the typical fantasy categories, what follows are PECOTA’s picks for the nine most productive hitters who will be 27 as of July 1, the cutoff point for player age notation across most baseball databases. They’re ranked according to their weighted mean forecasts for Equivalent Average, and using their PECOTA card data I’ve interpolated between the various underlying percentile forecasts to estimate both the chance that each player will reach a .300 EqA-a typically excellent season-and the chance that he’ll reach his career EqA.

Nate McLouth, Pirates CF
(.285/.365/.492, .296 EqA, Reach .300: 39%, Reach .286: 60%)
Poised to extend their record streak of futility into a 17th consecutive losing season, the Pirates at least have something to cheer about with McLouth topping this list. He broke out last year, ranking second only to Carlos Beltran in VORP among NL center fielders, and topping 20 homers and 20 steals, levels that PECOTA sees him as having a solid shot at reaching again. Despite winning a Gold Glove, his defense isn’t much to write home about, being valued at -4 Fielding Runs (FRAA) last year, and projected for -6 this year. With Andrew McCutchen on the way, he likely won’t occupy the middle pasture much longer, but his production is strong enough to survive the shift to a corner spot.

Adrian Gonzalez, Padres 1B
(.277/.356/.480, .295 EqA, Reach .300: 49%, Reach .299: 50%)
Gonzalez is doing his best to carry the mantle as the overall number one pick of the 2000 draft, never mind the fact that it was a horribly weak draft dominated by signability concerns, or that he’s two teams removed from the one that picked him. Toiling in the extremely pitcher-friendly Petco Park masks Gonzalez’s skill with the stick; he’s been extremely consistent over the past three years, with EqAs of .300, .300, and .311, and rates as having the highest chance among this lot to reach the .300 level again. PECOTA gives him less than a 10 percent chance of topping last year’s 36-homer outburst, but a 25 percent chance of reaching 30 dingers, by far the highest of anyone here.

Andre Ethier, Dodgers RF
(.292/.368/.464, .292 EqA, Reach .300: 36%, Reach .293: 50%)
Perhaps no Dodger should be smiling as widely as Ethier after Wednesday’s Manny Ramirez signing, as it returns the player whose arrival coincided with Ethier’s scorching .368/.448/.649 performance over the final two months, less likely due to any mythical protection effect than to the end of Joe Torre‘s dickering with the lineup’s Pierres and Joneses at Ethier’s expense. Ethier forecasts to have the highest EqOBP of any player here (.372), and assuming that he again hits ahead of Ramirez, that will put extra runs on the board for LA.

Kelly Johnson, Braves 2B
(.287/.370/.467, .290 EqA, Reach .300: 27%, Reach .281: 66%)

Ian Kinsler, Rangers 2B
(.284/.355/.472, .289 EqA, Reach .300: 33%, Reach .298: 34%)
While second basemen tend to peak earlier than players at any other position, PECOTA takes a rather oddly divergent view of this pair even after correcting for their hitting environments. Among this group, Johnson rates as having a relatively low chance of reaching a .300 EqA, but the best chance to top his career mark, while Kinsler has a better chance of reaching .300, but the list’s lowest chance of topping his career mark, suggesting that perhaps he’s already peaked. Kinsler is the faster player of the two, the more likely to put the ball in play, and the one who had the better 2008, trailing only Chase Utley and Dustin Pedroia in the VORP rankings for second base.

Mike Napoli, Angels C
(.240/.344/.482, .289 EqA, Reach .300: 34%, Reach .298: 38%)
Napoli comes straight from a favorite stathead mold: the rotund catcher who won’t hit for average, but who walks and wallops along the lines of Joe Ferguson (his #5 comparable), Gene Tenace (#10 comparable), Mickey Tettleton, and Mike Stanley. Fellow backstop Jeff Mathis‘ defensive superiority should push Napoli into some DH duty; there’s no excuse for a hitter this good to be getting fewer than 300 plate appearances in an offense that lacks the type of power and plate discipline he offers.

Corey Hart, Brewers RF
(.289/.343/.494, .286 EqA, Reach .300: 28%, Reach .277: 59%)
Hart went from being the second most valuable Brewer in 2007 to a basket case last year, hitting just .252/.275/.452 from June onward, and hacking his way into an 0-1 hole more often than any other player. PECOTA believes he can recover his plate discipline, and it gives him about a 25 percent chance at 25 home runs and 25 steals.

Conor Jackson, Diamondbacks 1B
(.295/.373/.461, .283 EqA, Reach .300: 23%, Reach .278: 50%)
Getting on base remains a major problem for the Diamondbacks, who squandered their NL West lead by going 51-55 with a .322 OBP from June 1 onward. Though Jackson lacks the power of a typical first baseman or left fielder-his spot depends somewhat on Eric Byrnesavailability and effectiveness-he forecasts to have the lineup’s highest OBP. Whether manager Bob Melvin realizes that he should be batting ahead of OBP sinkholes Stephen Drew and Chris Young remains to be seen, however.

Kevin Kouzmanoff, Padres 3B
(.275/.331/.465, .282 EqA, Reach .300: 22%, Reach .272: 63%)
Like Gonzalez, Kouzmanoff’s skills are masked by Petco, though the similarities end there; the latter lacks his teammate’s refined approach at the plate, as evidenced by his 6-to-1 strikeout-to-walk ratio. The late-season shoulder woes that Kouzmanoff played through forced his final line down to an ugly .260/.299/.433, but PECOTA’s fairly forgiving of his performance thanks to stronger 2006 and 2007 seasons, and it gives Kouzmanoff about a 15 percent chance at 30 homers, second only to Gonzalez.

                                       Reach Reach Career
Player            AVG/ OBP/ SLG   EqA  .300  Career  EqA
Nate McLouth     .285/.365/.492  .296   39%   60%   .286
Adrian Gonzalez  .277/.356/.480  .295   49%   50%   .299
Andre Ethier     .292/.368/.464  .292   36%   50%   .293
Kelly Johnson    .287/.370/.467  .290   27%   66%   .281
Ian Kinsler      .284/.355/.472  .289   33%   34%   .298
Mike Napoli      .240/.344/.482  .289   34%   38%   .298
Corey Hart       .289/.343/.494  .286   28%   59%   .277
Conor Jackson    .295/.373/.461  .283   23%   50%   .278
Kevin Kouzmanoff .275/.331/.465  .282   22%   63%   .272