In 2011, the average age for a Democrat in the House/Senate was 60.8 years old while the average Republican was 56 years old.
Compare that to ten years ago -- the 107th Congress -- when the average age was 55.7 for Democrats and 54.7 percent for Republicans.
And neither of those Congresses have anywhere close to the fresh-facedness (is that a word?) of the 1981 Congress when the average age for Democrats was 55.7 and for Republicans 54.7.
Obviously, large-scale seat gains/losses -- as happened during the Reagan revolution in 1980 -- tend to lower the average age of Congress as new blood sweeps in. But, even after Republicans picked up 63 House seats in the 2010 election the average age of all Members of Congress stayed relatively high as compared to historical standards.
There's no single reason to explain why Congress is in a period of historic old-ness.
One major one is that redistricting -- the national re-drawing of Congressional lines across the country -- has cemented the partisan nature of hundreds of districts across the country. What that means, from a practical political perspective, is that most Members -- regardless of their age -- have to work less hard to be re-elected and, therefore, are much more likely to keep running over (and over) again.
Another reason for the rising age of Congress is the death of the term limits movement -- if you are looking for an exact time of death, it's when Washington State Rep. George Nethercutt got re-elected in 2000 -- and the growing tendency to see politics as a profession. Politics is no longer regarded as a way station between careers or a sort of career-capper at its end. It's an end in itself. And that means people stay longer. Much longer.
The WSJ infographic is absolutely amazing -- allowing you to see the average age for Congresses dating all the way back to 1979. You can also see how many Members are a certain age and how they split between the two parties.