Man, this list got a lot worse up top this year -- thanks to the graduations of Manny Machado and Bryce Harper, among others -- since its most recent iteration. Nevertheless, the depth here goes well beyond 25 names and could easily run 40 deep before we're getting to guys who don't necessarily project as regular position players or average starting pitchers.
This list is limited to players whose seasonal ages for 2018 are 24 or younger -- that is, any player who will not yet have turned 25 by July 1, 2018 -- and who have exhausted their rookie status. That means no Harper or Machado (both are already 25) and no Ronald Acuna Jr. or Juan Soto (they haven't exceeded their rookie status yet).