Yesterday at Bill James Online (subscriber-only), Bill responded to a question about defensive reputations, and the possibility that they endure longer than hitting reputations, which is obviously true because when a hitter declines everybody can see the numbers. Fielders, not so much. But embedded in Bill's response is a pretty important piece of analysis that I think is little known ...
One of the first things that is becoming apparent from better defensive measurements is that defensive value peaks earlier and fades MUCH younger than offensive value. Players often reach their defensive peak at ages 22-25, and many players are fading defensively by the age of 28, long before 30.
I should stress that this is not new news, exactly. I don't know if it's in The Book (as usual, I've misplaced my copy), but co-author Mitchel Lichtman (a/k/a MGL) told me years ago that fielders peak in their early 20s. And I can promise you that Bill James isn't just swiping MGL's analysis.
As you probably know, hitters (as a group) peak in their late 20s.
Why would players peak differently as fielders and hitters? Well, this is rank speculation, but I will guess that it's because fielding is largely a matter of speed and reflexes, while hitting is largely a matter of strength, muscle memory and (yes) reflexes. Most hitters can't hit major-league pitches until they've seen thousands of them. But fielding in the majors is little different than fielding in the minors.
Some of this might seem counter-intuitive ... until you think about all the fielders who were brilliant from Day One.
One thing I'd like to know, though ... Do fielders at different positions peak at different ages? Stay tuned.