Dominic Raiola surprisingly played well in 2013 after a couple of down seasons. He just turned 35 though, so he'll regress quickly soon.
Jonathan Goodwin, C, 49ers. Age: 35. Signed with Saints
Jonathan Goodwin has told reporters that retirement has crossed his mind, but that didn't stop him from playing well in 2013. Goodwin will be 36 in December.
Joe Hawley, C, Falcons. Age: 25. Re-signed with Falcons (3 years)
Joe Hawley didn't start the first half of the 2013 season, but he took over the job in November and never looked back. He was solid, but unspectacular. Still, he's only 25, so there's plenty of room for improvement.
Rich Ohrnberger played very well in place of an injured Nick Hardwick in San Diego's playoff victory over Cincinnati.
Ryan Wendell, C, Patriots. Age: 28. Re-signed with Patriots (2 years)
Ryan Wendell had an excellent 2012 campaign, so his regression is a total mystery. He was so bad this year that he nearly got Tom Brady killed in several games.
Will Montgomery, C/G, Redskins. Age: 31. Signed with Broncos (1 year, $1.3 million)
Will Montgomery is decent at opening running lanes and has some versatility because he can also play guard. However, he's on the wrong side of 30 and was whistled for a ridiculous 11 penalties in 2013.
David Baas, C/G, Giants. Age: 32.
David Baas could be a solid backup because of his ability to play both guard and center. However, this would require him to stay healthy. Baas played just three games in 2013 and missed five more in 2011. He'll be 33 in September.
Brad Meester, C, Jaguars. Age: 37. Announced retirement
Brad Meester is nowhere near the player he once was, but he's still a serviceable center. He's certainly not to blame for Jacksonville's line woes.
Samson Satele, C, Colts. Age: 29. -- Signed with Dolphins
Kyle Cook, C, Bengals. Age: 31.
Ted Larsen, C, Buccaneers. Age: 27. -- Signed with Cardinals (2 years)
Fernando Velasco, C, Steelers. Age: 29.
Phil Costa, C, Cowboys. Age: 27. -- Signed with Colts
Chris Spencer, C, Titans. Age: 32. -- Re-signed with Titans
Joe Berger, C, Vikings. Age: 32. -- Re-signed with Vikings
Eric Olsen (RFA), C, Steelers. Age: 26. -- Signed with Titans