Bryce Harper has all season to move up on this list:
Most Career Regular Season Home Runs Before 21st Birthday
1. Mel Ott 61
2. Tony Conigliaro 56
3. Ken Griffey 38
4. Mickey Mantle 36
5. Frank Robinson 34
6. Al Kaline 32
T7. Bryce Harper, Alex Rodriguez and Andruw Jones 26
T10. Orlando Cepeda and Eddie Mathews 25