Adam Morris at Lone Star Ball has a terrific blog post on how good Elvis Andrus is at such a young age (spolier: very).â†µ
Andrus has already hit the four win mark in Wins Above Replacement at FanGraphs. 4.4 wins, to be exact. In short, it estimates he's been worth a little more than four more wins to the Rangers than a scrub filler (like Andres Blanco) would be at the same position for the same amount of time.
It may not sound like much, but four wins is usually good enough to have you in the top 70-80 players or so in baseball; what could roughly be called "All-Star caliber." He's done that already with a few games to go.â†µ
Perhaps encouragingly, as well, is that his defense is barely a factor in that total. Defense is a more speculative aspect of baseball to measure than offense, so when a player's high WAR is based largely on defense, it becomes a bit questionable. In Elvis's case, it only accounts for about half a win on his total, meaning he'd still be likely to hit the four win threshold with neutral defense. On top of that, we mostly expect his true talent to be higher than five runs saved in the field (above average, but not great); it's easy to see much more valuable years from Andrus on the immediate horizon without even much improvement on offense coupled with knocking off some of his error mistakes.â†µ
Most importantly, as Adam points out, the company of shortstops who are this good this young is pretty elite. He provides a list in the link above, and most of the players were at least very good for a while, with about half of the list being guys who are or should be in the Hall of Fame, or are off to pretty good starts.â†µ
Elvis Andrus is not just a nice young shortstop, he is having the kind of start that usually results in something special.