I know of one kid who broke into the majors at 19. Pitched a part-season -- 109 innings -- with a 1.88 ERA and 1.08 WHIP. Next season, 256 innings, 1.65 ERA, 0.96 WHIP, and HR/9 of 0.0. No kidding, zero homers in 256 innings. None. Nada.
Pitched for the Nats for about 20 years. At age 28 and age 31, again pitched full seasons without giving up a homer.
Conclusion: Giolito might break into the majors in 2015 or 2016 if he is as good as the greatest pitcher of all time...which seems unlikely.