This is about a Reds prospect, but I think it’s germane, and I found it interesting:
A very high percentage of the greatest players ever got to the majors when they were 20 or 19 and they did that in part because they had unusual talent, but it also works the other way. The guys get an opportunity to learn the game at the Major League level sometimes pick up things at a deeper level than the guys who have to learn the game at AA and AAA and relearn the game in the majors and sometimes unlearn a lot of stuff. There is a real problem with moving too slowly. On the other hand, in modern baseball, there’s usually a bigger problem with moving too fast. That’s in part because of the clock, particularly if you’re not one of the rich organizations, you’ve got six-and-a-half years to take advantage of the players you develop and you don’t want to waste too much of that while they’re figuring out how to play the game. You want to do as much as possible of figuring out how to play the game at the minor league level and then get the good years in the majors.
For what it’s worth, I checked, and it’s absolutely true about the greatest players getting to the majors at 19 or 20. Of the twenty top players in career Runs Created, sixteen got to the majors before their Age 21 season. (Admittedly, that gives them a better chance at a long career and to compile stats, but they’re the type of players you think of as the greatest in history.) Three of the four exceptions (Bonds, Palmeiro, Thomas) were college players who had short minor league careers. (And Bonds, of course, grew up around the game.) The other exception, Pete Rose, was unique in many ways and really not the type of player we’re talking about anyway.