This is not your father's National Football League—and in more ways than one.
With modern free agency comes the transience of NFL players that most now-generation fans and analysts have grown up with. More teams and more dollars create a labor landscape where more "better opportunities" lie with other teams, the grass often seeming truly greener on the other side (double entendre intended).
There are fewer men spending the entirety of their careers with their drafting organization. And, on a less ambitious scale, more players are failing to even reach five-, seven- or 10-year stays with their first teams, let alone subsequent employers.
Despite this cultural foundation in today's NFL, its antithesis is alive and well in the form of Detroit Lions placekicker Jason Hanson's career. If the fogie field-goal kicker can fend off University of Illinois undrafted rookie free agent Derek Dimke—no foregone conclusion given my rating of Dimke as the No. 2 kicking prospect in this past class—he will establish a new standard by appearing 21 seasons with the same club.
Have a look at this slideshow, where I touch on the longest-tenured (though not necessarily the oldest nor most experienced) player on every current NFL roster. I will be utilizing two great resources, Ourlads and Pro Football Focus, in bringing you some color to each guy's writeup.