There are currently two position players in baseball that are younger than 21: one of them, Mike Trout, is 20 years old; the other, Bryce Harper, is 19. Harper and Trout came in at #1 and #3, respectively, on Baseball America's top 100 prospects for 2012, as well as Minor League Ball's top 120 prospects. I need not explain that these are two extremely talented players who -- if they live up to the hype -- have marvelous careers ahead of them.
But it's worth noting that they've already accomplished a lot, just being in the majors already. In fact, if they can continue to be moderately productive for the rest of the season, they'll both put their names up alongside some pretty rare and special company.
Let's first examine the case of Trout. During the expansion era (since 1961), only 32 20-year-olds have appeared in 100+ games in a single season. Given that Trout has already appeared in 14 games, and that the Los Angeles Angels have 126 games left on their schedule, he's a pretty safe bet to reach that arbitrary mark of 100. That, alone, would put him in a unique class of players -- many of whom went on to produce at elite levels.
But let's expand the criteria. Let's look at pure age-20 production. Tossing away the playing time threshold, we get 31 position players -- in that 50+ year span -- who have topped 1 WAR at age 20. Guys like Alex Rodriguez, Johnny Bench, Roberto Alomar, et al. Trout is probably an even safer bet to join these ranks, as he currently sits at 0.6 WAR on the season.
It's hard to project a player like Trout -- a guy who isn't yet an established major-leaguer -- but a reasonable if pessimistic projection for the rest of the season would simply be his career numbers. In 54 games (196 plate appearances), he's hit .239/.301/.420 (103 OPS+), amassing a whopping 1.1 WAR -- in large part due to his great baserunning and fielding value. If Trout can merely continue along at the pace he's maintained throughout his entire career, and he comes to the plate another 450 times, that would put him at 2.5 WAR for the rest of the season. Add that to the 0.5 WAR he's already accumulated, and that would put him at 3.0 WAR for 2012 -- placing him 12th on the all-time list of 20-year-old players. That would put him right between Elvis Andrus and Joe Torre...not bad.
And given that Trout has gotten off to a scorching start (136 OPS+), this is perhaps a conservative estimate. Consider, for example, that ZiPS projects him to post a .337 wOBA over the rest of the season -- as compared to his career mark of .322.
Bryce Harper is an even more intriguing case, as that one year in age difference is very significant. If Harper reaches that 100-game threshold, he'd be eighth player to do that since 1961. Moreover, only a select few have actually been productive at that age. Eleven 19-year-old position players in that time span have topped 0.5 WAR. That's it. And interestingly enough, Trout is one of those guys.
Can Harper put himself in that company? I think so. He hasn't exactly raked since his call-up, but he's hitting .232/.313/.393. ZiPS projection system has him hitting .236/.314/.402 over 402 plate appearances the rest of the way, which would be worth 1.3 WAR (though do note that this is Fangraphs' implementation of WAR, as opposed to Baseball-Reference's). These projections should probably be taken with a grain of salt for a guy like Harper -- I'd presume he's prone to deviate greatly from these projections, in whatever direction. But just being worth 0.5 WAR would be historic -- and I think that's highly attainable for him.
Anyway, Trout and Harper have created a lot of buzz, and rightfully so. They're insanely talented players and it's going to be great to watch them blossom into stars over the next several years. But it's worth taking a step back and appreciating that, by just being here -- by being productive major-leaguers at such a young age -- they've already done (or rather, already will do) something very few others have done.