This list is limited to players whose seasonal ages for 2016 are 24 or under -- that is, any player who had yet to turn 25 on July 1, 2016 -- and players who have exhausted their rookie status. That means no Nolan Arenado or Wil Myers (already 25) and no Lucas Giolito or Jameson Taillon (haven't exceeded rookie status).
Note on the 2015 rank: NR stands for "not ranked." Any player who was a rookie or minor leaguer when Keith wrote last year's Top 25 under 25 (in late April) is tagged as such.
Most (Baseball-Reference) WAR through age-24 season by a position player (1901-2016):
1. Ty Cobb, 46.7 WAR
2. Mike Trout, 43.3
3. Mickey Mantle, 40.9
End of message.
Harper is having a down year compared to his phenomenal 2015 season, which was one of the best seasons in MLB history by a player his age (22) or younger, but he's in the top five in the National League in OBP and the top 15 in home runs, so his "down" year is still quite productive. And, of course, we've seen what his up years are like. As much as people like -- or liked, before last year -- to call Harper overrated, he's doing things before age 25 that very few players in MLB history have done. Well, few other than Trout.