Like woy said, Ott's the standard for teenagers. Foxx only had 147 plate appearances during his age 19 season so he didn't really have a large enough sample.
Here's the list of teens with a RC/27 ratio higher than 100 (league average) with a minimum 300 plate appearances. The list is rather small and is littered with Hall of Famers (and a few guys like Magee and Sheckard who probably should be in the Hall).
Code:SEASON AGE < 20 RCAA displayed only--not a sorting criteria PLATE APPEARANCES displayed only--not a sorting criteria AGE displayed only--not a sorting criteria RUNS CREATED/GAME YEAR RATE PLAYER LEAGUE RCAA PA AGE 1 Mel Ott 1928 146 7.76 5.30 29 499 19 2 Tony Conigliaro 1964 143 6.35 4.43 19 444 19 3 Ty Cobb 1906 142 5.92 4.17 12 394 19 4 Sherry Magee 1904 116 5.10 4.40 8 387 19 5 Mickey Mantle 1951 114 5.67 4.95 13 386 19 6 Ken Griffey Jr. 1989 114 4.92 4.30 6 506 19 7 Monte Ward 1879 112 5.80 5.17 8 371 19 8 Cesar Cedeno 1970 112 5.46 4.89 9 377 19 9 Jimmy Sheckard 1898 109 5.70 5.21 8 459 19 10 Edgar Renteria 1996 104 5.28 5.06 6 471 19 11 Joe Quinn 1884 102 5.65 5.53 3 438 19 12 George Davis 1890 101 6.11 6.07 4 583 19