In his age 23 season (next year for Castro), Ernie Banks made 34 errors. Eventually, he did move from shortstop... seven years later, and oddly, began the transition the year after winning the GG. Just for reference sake, I went and looked at some past winners of the NL GG: Castro at age 21: 158 G, 29 E, .961% Age 22 season: (Games/Errors/Fielding %) Renteria: 151, 26, .959% O. Cabrera: 117, 28, .944% (Minors) Larkin: 91, 17, .963% (Minors) (At age 24, in the majors: 148, 29, .960%) Concepcion: 93, 22, .945%