In the past century, the average retirement age has been steadily decreasing up until very recently when it began going up again[1]. People spend more time in school earlier, but education usually leads to working later in life as well[2].

[1] http://www.bls.gov/mlr/1992/07/art3full.pdf [2] http://crr.bc.edu/wp-content/uploads/2011/08/IB_11-11-508.pd...