This is Josh Fields' minor league translated WAR150. His performance from his '04-'08 minor league seasons have been extrapolated out to 150 games, with defense eyeballed using his MLB UZR and minor league Total Zone ratings (which can be found on minor league splits). His age 26 season is what he actually did in the MLB this year, more or less.
This is really just an excuse to point out Dan Szymborski's recently released minor league translations that go back 30 years. They are very cool. As larry noted, some prospects simply reach their ceiling early and fall by the wayside. Knowing what we know now, Fields' probably ought to have been traded following '07. It was far less clear at the time, obviously. Just for kicks, here's Mike Cameron.
Again, the final season is actual major league performance. I actually just plugged in +5 fielding runs above average, but really there's a good chance he was much better. UZR only goes back to '02, his age 29 season. Fielding skill peaks from 24-28 or so, and it may be younger in the OF where it's almost all about how fast your legs can carry you. From '02 to '09, he's averaged 4 WAR per season, the level of play he established in his age 23 and 24 seasons. Nothing against Paulie, but I'm pretty sure the Sox lost that trade. I was all of 12 when it was made, so maybe there's something I'm missing.