Here is A LINK TO A Cool stat analysis story by Cyril Morong...
Which players improved the most and declined the most in 2006?
The analysis is that you can tell someone is about to really just lose it all if they hit a deep decline and are in their mid-30s. Makes sense.
Conversely, while some seasons of improvement are blips or spikes in an otherwise non-progressive career, if a player is in that magic quadrant of being 26 - 29 years old, a spike may only be the first or second step up to the "new level"...
So Juan Rivera (who turned 28 midseason) had the 6th biggest offensive improvement in baseball among players with over 300 Plate Appearances.
And in 7th place? Adam LaRoche, who just turned 27.
No other Angels besides Rivera were in the Top 25, and none were in the Bottom 25...
(although Mark Ellis and Bobby Crosby were!)