This list is limited to players whose seasonal ages for 2015 are 24 and under -- that is, any player who has yet to turn 25 by July 1, 2015 -- and to players who had already exhausted their rookie status before Opening Day. That means no Giancarlo Stanton (he's already 25) or Anthony Rendon (he turns 25 in June), no Kris Bryant (still a rookie) or Jorge Soler (he didn't pass the Rookie of the Year limits until after Opening Day).
One trick I run into doing this list in late April every year is the presence of enormous piles of small samples of data in front of me, just daring me to overreact to them. I mention them in most comments, with the necessary disclaimer that they might be nothing more than noise, but in a few cases the changes look like they may be significant enough to endure. And no, I didn't intend to put three players from one club in consecutive slots. It just worked out that way.
Please contain your surprise. Trout's 29.4 WAR (per Baseball-Reference) is the sixth-highest total ever for any player through his age-23 season, behind four Hall of Famers and Ken Griffey Jr., and Trout racked up his total in 74 fewer games than any of those other players had. Even his new approach this year, being more aggressive early in the count, hasn't dinged his value; he's walking as often as before, striking out less and still hitting for average and power.
Other players can call Harper "overrated" all they want, but it reeks of envy, as Harper has been one of the most productive players for his age in MLB history despite playing around an injury that sapped some of his hand strength last season. He's already leading the National League in walks in 2015 and is in a giant tie for third place in home runs, while offering above-average defense in right -- and he'll still qualify for this list for another two seasons.