https://research.stlouisfed.org/fred2/series/CIVPART

Labor force participation rate being at 62.8% is still above what it was in the 50s, 60s & early to mid 70s. The primary factor for it going up starting in the 70's was due to the baby boomer generation entering the work force.  The 2 major demographic factors for it going down since 2008, besides the "Great Recession" of the Bush2 Administration has been: 1) the increasing numbers of baby boomers retiring and 2) the shift towards a more automated work force.