5. Fernando Valenzuela
Years Active: 1980-1991, 1993-1997
Team(s): Dodgers, Angels, Orioles, Phillies, Padres, Cardinals
Playing Weight: 180 lbs.
He may not have the career numbers of some of the players lower on this list, but they didn't have a mania associated with their debuts. Yet another big ballplayer to appear in the majors before his 20th birthday, Valenzuela more than justified the "Fernando Mania" that gripped L.A. during his rookie year in 1981. He won the Cy Young and Rookie of the Year awards that year (with a Silver Slugger for good measure), leading the league in complete games, strikeouts, and innings pitched. He was an All-Star his first six years in the bigs, and led the NL in wins with 21 in 1986. Not bad for a guy who didn't even look at the plate when he pitched!