Bryce Harper is thus far the only player born in 1992 to reach the majors. His current Wins Above Replacement (WAR) (Baseball-reference version) career total of 1.4 is thus tops by default so far for his birth year.
Five players born in 1991 have reached the majors, but Mike Trout is the only one with positive WAR. At 4.8 career WAR with half a season still to play, Trout is on pace to have one of the great very-young-career starts ever, according to b-ref’s WAR. The only MLB hitters in history to reach more than 6.5 career WAR through the end of his age 20 season have been Ott, Cobb, Kaline, A-Rod, Mantle, Junior Griffey and Ted Williams.
Among players born in 1990, b-ref has Brett Lawrie as the career WAR leader with 8.3 WAR, ahead of Starlin Castro at 7.2. Fangraphs, however, has Castro ahead with 7.6 to Lawrie’s 5.5. Averaging b-ref and fangraphs puts Castro ahead 7.4 to 6.9.
More year-of-birth WAR leaders after the jump.
Most Career WAR (baseball-reference version), active MLB players by year of birth:
Born 1989: Jason Heyward, 11.9 (G.C.M. Stanton is a strong second at 9.5)
Born 1988: Clayton Kershaw 20.1 (Elvis Andrus is second at 9.3)
Born 1987: Austin Jackson 12.6 (Justin Upton is second at 11.7 — but fangraphs has Upton ahead of Jackson, 15.6 to 10.4)
Born 1986: Felix Hernandez 28.6 (McCutcheon, Sandoval and Wieters, in that order, are the leaders among the batters)
Born 1985: Evan Longoria 27.6
Born 1984: Matt Cain 28.0
Born 1983: Miguel Cabrera 40.4
Born 1982: David Wright 36.0
Born 1981: Carlos Zambrano 36.7
Born 1980: Albert Pujols 85.6
Born 1979: Adrian Beltre 56.3
Born 1978: Chase Utley 50.4
Born 1977: Roy Halladay 63.0 (just ahead of Carlos Beltran at 61.2)
Born 1976: Lance Berkman 48.9
Born 1975: Alex Rodriguez 111.5
Born 1974: Derek Jeter 68.3
Born 1973: Todd Helton 58.6
Born 1972: Chipper Jones 80.3
Born 1971: Jason Giambi 47.8 (retired players who were born in 1971 and accumulated higher career WAR than Jason: Pedro Martinez, I-Rod, Brian Giles)
Born 1970: Jim Thome 67.6
Born 1969: Mariano Rivera 52.7 (Ken Griffey, Jr., also born in 1969, reached 79.2 WAR)
The only players born before 1969 who have been active in the majors in 2012 are Omar Vizquel (born 1967, career WAR 40.8) and Jamie Moyer (born 1962, career WAR 44.8).