The extreme left Economic Policy Institute here puts the number of missing workers in the US employment statistics at almost 6 million, give or take, since October 2013.
This is premised on a 2007 government projection of labor force participation rates in a healthy economy to 2016 which already controlled for demographic change, including the burgeoning retirements of Baby Boomers and, evidently, emerging smaller workforce cohorts due to declining fertility rates. That can be used against the measurements of labor force participation rates and labor force levels which have actually filled the record over the period to derive discrepancies.
Currently EPI calculates the discrepancies to mean that unemployment is 9.7%, not 6.3%.
And you thought only wild-eyed kooks on the right thought unemployment was still that high.