Happy 21st birthday, Bryce Harper!
In two seasons to date, Harper has posted a 128 wRC+ while hitting .272/.353/.481 in 1094 plate appearances.
Steamer projections at fangraphs.com have Harper projected to hit .266/.347/.464 as a 21-year old. This seems quite low, and it wouldn't surprise me if that system, which I don't know much about, is simply confused by young talents like Harper. Perhaps it uses an average 21-year old's line, which must be rather poor, as part of its projections? I don't know; maybe someone can enlighten me.
Either way, if Harper posts a lower batting average, OBP, and slugging than he did in either of his first two years, I imagine that would be a major disappointment, not just for fans of the Washington Nationals, but for fans of the sport of baseball. And also, perhaps mostly, for the player himself. (But at least the projections have him down for a career-high 23 home runs.)
Changing gears for a moment, how about that Mike...Salmon, was it? No, Mike Trout. You may have heard, but some people thought he was the American League's most valuable player after he hit .326/.399/.564 in 639 plate appearances in 2012. Then he somehow got even better as a hitter in 2013, posting a .323/.432/.557 line in 716 PA.
But when Trout was 19, he hit .220/.281/.390 in a 40-game, 135-PA cameo in 2011. Harper would crush that line as a 19-year old rookie in 2012. Then, of course, Trout's age-20 season set an impossible standard that Harper had about a 0.000000034% chance of surpassing, if we're being optimistic.
Because of the one-year age difference, had Trout just ended up reasonably good rather than ridiculously great, he might have served as a decent guide for how Harper might develop. But Trout's 2013 season confirmed that he is ridiculously great, so that idea is out the window for now.
What about other players who got their starts as teenagers? According to the baseball-reference.com similarity scores, Harper through his age 20 season has posted numbers most similar to Tony Conigliaro (956), Ken Griffey (954) and Mickey Mantle (954). All three of these players debuted in their age 19 seasons.
Mantle was already a great hitter when he was 20, posting a .311/.394/.530 line in 626 PA (158 wRC+), but the other two players set more worldly, but still great-for-20, lines: Griffey a .300/.366/.481 (666 PA, 132 wRC+) and Conigliaro a .269/.338/.512 (586 PA, 131 wRC+).
Harper's wRC+ in 2013 was 137, slightly better than either Griffey or Conigliaro, but he only put in 497 plate appearances. Still, the three players had awfully similar age-20 seasons.
When he turned 21, Conigliaro's effectiveness decreased to a 123 wRC+ and .265/.330/.487, before a recovery when he turned 22 (144 wRC+, .287/.341/.519, 389 PA) prior to the disaster that occurred on August 18, 1967, when he was hit in the face by a pitch.
Griffey's improvement was steadier, as he posted a .327/.399/.527 line when he was 21 and a .308/.361/.535 one at 22 years old, with wRC+ marks of 148 and 145, before experiencing his first two 160 wRC+ seasons the next two years.
One more player I want to talk about in this context is Giancarlo Stanton. He fiddled around in A+ and AA when he was 19, because the universe doesn't just up and grant every great talent the ability to hit Major Leage pitching as a teenager. Stanton instead debuted in his age 20 season and hit .259/.326/.507 (118 wRC+ in 396 PA) before hitting 34 home runs in his age 21 season with a 141 wRC+ and a .262/.356/.537 line in 601 PA.
So where the heck are we now? I just shared a lot of names and players and numbers and slashes, but none belong to Bryce Harper. He'll have a heck of a lot more to do with his development than Mickey Mantle's ghost.
I think the record shows, however, that players who hit well when they are 19 and 20 generally don't stagnate at 21. The projected line from the beginning of this post still seems low.
To conclude, here is a possible range of outcomes for Bryce Harper in 2014:
Worst-case: His health remains an issue. His stats end up about as projected...or worse.
Mid-case #1: He actually gets healthy but still faces a Conigliaro-like decline between his age 20 and age 21 seasons. (Although, that Conigliaro's decline still left him hitting at a darned good level.)
The Steamer projection is somewhere between this and the prior case.
Mid-case #2: Ken Griffey. Don't let the version of Ken Griffey from his mid-20s in the mid-90s, the version who hit 56 home runs in consecutive seasons, interfere with the classification of this as a "mid-case." A 10-20 point jump in Harper's wRC+, as Griffey experienced when he turned 21, would be a welcome development and continue Harper on his perennial all-star path.
Best-case: Mike Trout. I might have skipped a couple mid-cases, but let's get back to Trout. It's going to always get back to Trout, I think, for years when we have conversations like this. But if Trout could struggle when he was 19--unlike Harper, Mantle, Conigliaro, Griffey (sorry Stanton)--and then explode when he turned 20, why can't the other once-in-a-generation talent of this generation experience a similar jump? (Please allow me a "why can't" when talking about best-case scenarios.) It wouldn't be a change from bad to great, but good to unfathomable, and it would come a year later, but maybe instead of having Griffey's age-20 season and Griffey's age-21 season, Harper can skip right to Trout or Mantle's age-21 season.
"Griffey-Griffey" is still a more realistic hope for those looking for Harper to exceed the computed expectations set by Steamer. I don't think a 150 wRC+ is out of reach, but even a 140 or 145 wRC+ or so would be a nice continuation for Harper's career.