How long is too long for an O-line?

In a recent mailbag column, a reader expressed concern about the age of the Detroit Lions' offensive line.

That gave me the opportunity to bring back "The Theory of 150," a concept I stumbled upon in 2007. That year, the Chicago Bears were trying to squeeze an extra year out of an aging offensive line. Coming off a trip to the Super Bowl, the gamble seemed reasonable.

Bears general manager Jerry Angelo told me at training camp then that it didn't make sense to break up the line in 2007. It was worth one more season. Tackle John Tait was 32. Tackle Fred Miller was 34. Center Olin Kreutz was 30. Guard Ruben Brown was 35. Guard Roberto Garza was 28. Total age: 159. Average age: 31.8.

Father Time won that season. The Bears went 7-9. Brown lasted only eight starts. The line gave up 43 sacks, and the team averaged only 83.1 rushing yards per game and a feeble 3.1 yards per carry.

That experience caused me to study offensive line age. Sure enough, once a starting line reaches the combined age total of 150 and has three to five starters in their 30s, prepare for drop-offs.

The 2008 Washington Redskins line reached 158 in 2007 and 161 in 2008. The Redskins got a wild-card playoff trip in 2007, but in 2008, they went from 29 sacks allowed to 38. Since then, the Redskins' line has given up an average of 44.3 sacks, and the rushing average has hovered between 3.9 and 4.22 per carry.

The 2009 Dallas Cowboys' offensive line hit the 157 mark and had five starters in their 30s. Problems followed, and the five starters -- Kyle Kosier, Flozell Adams, Marc Colombo, Leonard Davis and Andre Gurode -- are not only gone but are currently out of the league.

In the past couple of years, the New York Giants and the New England Patriots have come close to the 150 threshold, causing Tom Coughlin, Bill Belichick and their personnel offices to scramble.

The Giants and the Seattle Seahawks proved in the mid-to-late 2000s that if you can keep a starting offensive line together for 50 starts, blocking chemistry becomes so consistent that average talents become top players. Communication on such experienced lines becomes instinctual.

But what happens if the line stays together too long? It takes years to return to past glory.

That came into sharp focus when I looked at the 2011 Pro Football Focus ratings of offensive lines. Three of the six lowest-rated lines last year were affected by the Theory of 150: the Bears (32nd), Giants (31st) and Redskins (27th).

The Patriots started to address the problem in 2009, when they drafted tackle Sebastian Vollmer. In 2011, they used a first-round pick on Nate Solder. They were at 151 in 2009, so the semi-youth movement has helped keep them at No. 3 for blocking, according to Pro Football Focus.

Selecting Tyron Smith in the first round last year re-energized the Cowboys' line, and he moves to left tackle this year. That should help Doug Free, who struggled at left tackle last season and will move back to right tackle. The running offense has struggled -- hovering around 112 yards per game. Sacks are in the 30s. Pro Football Focus rated the Cowboys 15th last year, which isn't bad considering 2009.

Guards start wearing down once they get to 32. Letting a guard hang around when he's 34 is stretching it. Centers can go into their mid-30s, but they do risk injuries.

Which brings us back to the Lions. They currently have the league's second-oldest line -- an average age of 30.6 and a total number of 153, which is a concern. They made a good move a couple years ago trading for guard Rob Sims, who might have been their best blocker last year. They used a first-round pick on Riley Reiff to replace Jeff Backus, who is starting but is 34.

Even though the Lions had a great season and made the playoffs, they averaged only 95.2 rushing yards a game, partly because of injuries, and they surrendered 36 sacks.

Another team that needs to worry is Baltimore. If the Ravens start Bryant McKinnie (32 and overweight) at left tackle, Matt Birk (35) at center and the recently signed Bobbie Williams (35) at left guard, Baltimore will have the oldest starting line at 155.

The Tennessee Titans also need to be concerned about possible drop-offs along their offensive line. They signed 34-year-old Steve Hutchinson to stabilize the left side of the line, but that took their starting age total to 150. Titans coach Mike Munchak has been trying to replace 30-year-old starter Eugene Amano at center.

Tackles Michael Roos and David Stewart are 29 and among the best at their positions, but the clock may start ticking on them soon. The interior of the line is the real source of worry.

The work isn't over for the Giants, who didn't re-sign Kareem McKenzie at right tackle and have major issues at both tackle positions.

Keeping lines together is great for continuity, but no team can let the clock run out.