We've read dozens, maybe 100, answers like the below from Bill James. James is the guy who works in a "peer-reviewed journal" context, as it were (as opposed to working in an echo chamber). Sabes are welcome to challenge his "Meta-Study" mind and orientation, but they almost never do. That's important. James is a reasonable substitution for a general consensus. That's rare, but it happens.
Anyway, a sample answer in Hey Bill:
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I seem to remember you writing that there was a strong correlation between the age someone established himself as a major league player and greatness. Assuming that I haven't totally distorted what you were saying, who is having the more impressive season: Trout or Harper? In Harper's favor is that Trout could not handle major league pitching at age 19. The argument for Trout is that he's made such a leap forward that Harper is unlikely to be able to improve that much in his age 20 season.
Asked by: Hank Gillette
Answered: 7/5/2012
Well, the reason that the age a player gets established in the majors correlates strongly with greatness is that (1) players have to work themselves up from level to level to level, and (2) the odds are most favorable for younger players. Considering just (1) first. ...Suppose that you have 1000 players at the level of "Regular Major League Player", and that a player must step forward four times to become a major league player (Good Regular Player, Marginal All-Star, Perennial All-Star, All-Time Great.) Suppose that 30% of players take a step forward each year. If you start player out at age 25, almost none of them will make four steps forward by age 30. But if you start them out at age 21, some of them will.
Factor (2) is that if that percentage that steps forward is 30% at age 25, it's 40% or 45% at age 21. It is never true that MOST players take a step forward or that players can be counted on to take a step forward. Many young players never improve very much, like Alex Rios and. . .what's his name, the Upton who plays center for Tampa Bay. They reach a certain level; that's just where they are. It's really unfair to those players to EXPECT them to be something that they're not.
But if a player gets established at age 21, then he has a lot more time to consolidate his gains and take another step forward.
Trading off the age 21 vs. age 19 and one level of performance vs. another level of performance, putting that all into one sausage grinder. . .well, that's complicated math, and I wouldn't have a lot of confidence in my ability to do it well or anybody else's, frankly. Al Kaline was a great player, but he was never really any greater than he was when he was 20 and 21 years old, or not much greater anyway. Ted Williams was never greater than he was at 22. Young players SOMETIMES make explosive steps forward in ways that players almost never do after the age of 25.
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In another answer, Bill summarized that the GROUP of established 23-year-old hitters are probably about 90% of what they'll be at their peak. Figure on maybe 100 points of OPS+ incline from there.
But that is for the group, and it is presuming that the 23-year-olds have already adjusted to the league, had stock minor league backgrounds, etc etc. Some 23-year-olds are late bloomers, obviously; it is why teams with no resources "gamble on young players improving" rather than going to the FA market.
There are any number of Carlos Guillens, Alex Gordons, Raul Ibanezes, etc who are not sheep in the middle of the age-curve herd. It is not illogical to believe that Player X is something other than ordinary.
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On catchers, James originally said that their age-arc slides +2 years later, but he has recently said it may be more like +1.5 years. Zunino's curve may be skewed (either way) by the weird lack of prep time.
A few minutes' clicking around will find tons of catchers who hit much better at age 28 than at age 23. (And some who wash out.)
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As McClendon says, Zunino is "just a pup" and as the almanacs say, Dustin Ackley was a #2 overall. Alex Gordon OPS+'ed 90 at age 23, but 140 at age 27.
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In any case, beware the man who believes he has 970 of 1,000 light bulbs on. There are actually billions or trillions of light bulbs out there.
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