The Census Bureau recently released the results of the 2011 American Housing Survey. One noteworthy point for me was how old housing is in the U.S. The data basically shows a rough bell curve peaking between 1950 and 1979. The median year was 1974, 34 years ago. This number has crept up since 1985, when the median house age was only 23, according to the National Association of Home Builders.
For more analysis on this issue, check out this oldhouseweb.com article from a few years ago. It shows which states had the highest concentration of old housing at that time. A surprising amount of it is in the Midwest, with Michigan, Illinois, Wisconsin, Indiana, and Ohio all in the top ten.