Back in December of last year I went through some calculations with the data set I stitched together including both performance and salary data. You can check the post out here for a series of simple graphs that go a long way toward explaining team behaviors and why certain things are the way they are in the NFL, particularly as they relate to player tenure and performance over time.
As I was reading through that post again recently, however, I was struck that it makes the implicit assumption (explicit above) that things are the way they are. What I mean is that this data is the average over a period of time, while there may in fact be some trends to observe by looking at the period year-by-year.
The average age of all NFL players fluctuated between 26.5 and 27 for most of the post-1994 period (the salary cap era) before dropping from 2008 through 2013 almost without interruption. Applying a weight based on games played or games started, the other two lines on the chart, indicate that the pattern is consistent for the starters as well as the backups. NFL rosters are roughly half a year younger than they were from 94-08.