This is a NerdGraphs leaderboard of the results of all 20-year old players from 1900-2012 (baseball's "modern era"). Small sample size caveat, obviously... but just look at where his OPS puts him historically based on his start this year.
To stay up that far on this list, he'd need to continue raking the way he has all year and adjust to pitchers as much as they'll adjust to him. As you look at the names in that list, though, the thing that stands out is that there just aren't many players that put up the kind of stats he's putting up at this age. The vast majority of those went on to become superstars and Hall of Famers. It's good company to be in.
FYI: This link can be bookmarked to check in on it all year long. I know I will be.