Economist's View recently flagged a San Francisco Fed analysis of unemployment rates here.
Some key points:
-labor force participation was at a historical high in 2000.
-"Labor force participation rates are highest for those aged 25-54; after age 55, labor force participation rates fall continuously as individuals retire. Thus, the aggregate U.S. labor force participation rate is expected to fall as the baby boom generation ages and retires."
-"Participation rates of men 25-54 have been gradually falling for more than 40 years." I see this as a result of the entrance of women into the workforce. I think there are a significant number of men in this age range either living with their parents or living off their wives' pay.