An NFL roster should have a good balance between young hungry players, and grizzled veteran leadership. You have too many young guys and your roster lacks the experience necessary to deal with the peaks and valleys of a typical NFL season. But too many older guys, and your roster is more than likely to spend too much time in the trainers room.
Professional football is a still a young mans game, but the depth of a roster should be full of ascending talent, ready to push the veterans for starting spots.
The Chicago Bears have had one of the oldest rosters in the NFL for a few years now and they've seen their win total decrease from 10 in 2012, to 8 in 2013 and 5 in 2014.