Grantland: Keri: Middle Relief: Rating the Rookie Eras

By expanding our query to players 25 and younger, we can check in on multiple eras to see what Trout, Harper, & Co. are up against. Thanks to Baseball-Reference’s Play Index, and with an assist from ESPN Stats & Info’s Mark Simon, we find five different years in baseball history where 14 24-and-younger future Hall of Famers were playing at once: Every season from 1923 through 1926, as well as 1956. Going by that standard gets tricky, though, since some of those players went on to make the Hall of Fame as managers. Using hunt-and-peck tactics, we can pinpoint some of the best 25-and-under generational boomlets of all time. ...

For sheer volume of greatness, 1955 might be it. Under-25 everyday players included Hank Aaron, Willie Mays, Mickey Mantle, Roberto Clemente, Ernie Banks, Al Kaline, and Eddie Mathews, while Harmon Killebrew and Frank Robinson were starting out. Yikes. Young pitching stars were scarcer, though future Hall of Famer Early Wynn fit the bill. There was also a terribly wild, 19-year-old lefty just getting his feet wet. It would be six years before he’d have his first great season. But anytime you can include Sandy Koufax on any list like this, you just do it. Unless Yoenis Cespedes is the next Clemente and Todd Frazier the next Mathews, you’re not touching this group.