The hallmark of the Chicago Bears for years has been defensive excellence. Last season was no different in that regard, as they had the best defense in the league according to Football Outsiders' measure of team-efficiency, defense-adjusted value over average (DVOA, explained here).
It was the eighth time in the past nine seasons the Bears ranked as one of the top 10 defenses in the league by DVOA. That is an eternity in the current NFL, where personnel turnover is rampant and a quality defense is much harder to sustain from one year to the next than a proficient offense.
For years, Chicago's defense has been headlined by players like Julius Peppers, Brian Urlacher, Lance Briggs and Charles Tillman. Urlacher has now retired, but Peppers, Briggs and Tillman remain. Unfortunately, all three are on the wrong side of 30. Are the Bears now too old to continue to sustain a great defense?