Since 2008 there are 11 pitchers currently season-age 27 or younger that have pitched at least 690 innings with an ERA <4.00.
They are :
(I'm not saying John is as good as these guys. All have pitched at least 50 more innings and 8 out of the "other" 10 have significantly better career ERAs. However I do think that John's accomplishment of pitching so many quality innings, even if the fact that those innings ended up as quality is inexplicable to some, at such a young age is kind of big deal. Can it happen with average pitchers? Sure. Going back to 1994 you find Bobby Jones did it. Joey Hamilton. But the vast majority of guys that are able to do this are very good to great pitchers. In other words - it's not easy to do at all.)