When you're evaluating the performance of any player -- rookie, veteran, or prospect -- age is almost always a meaningful data point. For rookies and veterans we're usually interested in proximity to their prime years (forwards or backwards); with some veterans we're also concerned with advancing age and inevitable decline (this is the corollary to prime age). Prospect rankings are always made with respect to age-versus-level; a young player struggling in Triple-A is usually a better prospect than an older player dominating Low-A.
Where age is concerned, the Mets of the aughts have been generally, well, elderly. The Tom Glavines, Moises Alous, and Julio Francos played no small part in that trend. The Mets do seem to be getting younger, and friend of the site Chris McShane did a bit of legwork to put together some graphs of the Mets' average batter and pitcher age over the past half-decade. Thanks also to Jeff Zimmerman, who provided historical league average pitcher/batter ages (as he used in this post at Beyond The Boxscore) which Chris incorporated as a baseline in his graphs. Jeff's data only ran through 2008; the numbers used for 2009 are from Baseball Reference's weighted averages for age, which, in short, are based upon age and playing time. The 2010 estimates use the same formulas and CHONE projections for the expected Mets roster.
The Mets' average pitcher age has dropped by almost three years since 2006; their average batter age has dropped by a year-and-a-half since 2007. Perhaps most telling is that the Mets are *still* above the league average in both categories despite the dip over recent seasons.
I'm not sure what meaningful conclusions you can draw from this data. The Mets were hella old in 2006, but the team was terrific. If anything, there's a semi-clear correlative relationship between declining average age and waning team success. We're only talking about one team over four or five years, so again there isn't anything truly remarkable here, but the data are interesting nonetheless, as I'm personally happy to see the team getting a bit younger.