1 - Labor Force Participation Rate, US Oct 2016 (62.8%: anyone over 16, not including your grand daughter or son... raw is 57.9 btw) + US Unemployment rate (4.9%) = 42.1%
Removing the 65+ years age rate (13.5%) lowers it to 28.6%.
This slightly lowers the rate, since a portion of the 65+ is working, but I am too lazy to look that up. But, 28.6% of the population above 16 and below 65 are not employed. Doesn't take in effect stay-at-homes, disabled, etc... Yet, I do not like the U-6, since it only considers those not working but say that they are willing to work and have looked for work in the last 12 months. It eliminates the long-term discouraged. However, isn't that a surprising number, variance of 28.6 to 9.5%.
Are you saying that 19.1% of the potential workforce are disabled, stay-at-home, etc? I can't find a good translational metric for stay at home (29% of mothers but not a good test for current population.. how many are applicable mothers) or disability that would apply, especially since I am slapping this together.
FYI - Underemployment rate is 12.9%
Source is the BLS and the Census