Alan Schwarz looked at this table and concluded that rookies are "getting consistently older, not younger" when they make their first appearances in an MLB game.
If we just consider the last row (TOTAL), the average age by decade regardless of playing positions, his conclusion makes sense, at least from the 1960s to the 2000s.
Alan went a step further, providing data to help interpret the average age based on the positions of the rookies. The nine additional rows contain 54 more numbers, with values all within a narrow range (22-26). How should the reader interpret these numbers? The following chart lends some help. The black line depicts the average debut age over all positions and is identical in each graph, establishing a standard for comparison. (The vertical axis plots average age at debut.)
Here are some key insights:
- Age getting older is shown by the black line. The green lines show that this conclusion does not apply to every positions: for example, the age of rookie shortstops (SS) appears to have fallen.
- Rookie shortstops (SS) and center-fielders (CF) have always debuted at younger ages than average. Same for right-fielders (RF) until the 2000s.
- On the other hand, catchers (C) and to lesser extents, left-fielders (LF) and pitchers (P) have debuted at older ages than average.
- Specific deviations from the norm are revealed: the shortstop (SS) curve is clearly at odds with everything else; rookie 1st basemen (1B) have debuted at older ages in the 2000s than before.
All these points can be obtained from the table of numbers as well; it just takes much more time and much more effort than looking at a chart.
Reference: "For Baseball Rookies, the Only Rush Is to Judgment", New York Times, Sept 4 2005.