So how much better can Mike Trout be?
Here are Mike Trout's wRC+ in his first four full seasons:
167 176 167 174
In terms of runs and fundamental performance, there's essentially no difference between any of those numbers. He's held almost exactly steady.
Still, I was recently asked if Mike Trout can get any better.
My answer was that no, he probably can't. Because there's just so little room for improvement. Or so my reasoning went.
But there's a better reason to believe that Trout won't improve, or at least not much, and it's a reason that I should have already internalized: As Jeff Zimmerman pointed out a couple of years ago, the "aging curve" isn't what we used to think it was. Zimmerman's nut graf:
With wRC+, the most recent aging curve doesn’t immediately begin declining like the wOBA curve. Instead, it remains constant until it begins to decline. The decline starts at the same point when previous players began declining (between age 25 to 26 season). The curve shape is the same for pitcher aging curves: no up and down, just constant and then down. Additionally, the most recent rate of decline is almost the same as the pre-PED aging rate (82-89).
Does this mean that baseball players don't improve as they get older?
No it doesn't mean that!
Mike Trout was undoubtedly better at 21 than 16. What it means that if a player's actually good enough to play regularly in the majors, he's approximately as good as he'll ever be. On average. There are dozens (hundreds?) of 20-year-old players in Class A right now who will keep better until they reach the majors at 22 or 23 or 24 ... but most of them won't improve much after that.
If you believe Zimmerman's conclusions, anyway. The point is that players get promoted to the majors as soon as they're good enough -- again, generally speaking -- but almost as soon as they're good enough, their physical abilities have essentially plateaued.
So, no: Mike Trout's not getting any better. But he's still so young that his decline phase doesn't begin for another year or two. And even then, he'll still probably be the best player in the league for another three or four years.