The BLS only counts the people who aren't in prison, and are over 16 as the Labor Force participation rate. You aren't counted if you are thought of by employers as too young or too old to work; so there are a substantial number of seniors who aren't counted because they aren't in the labor force according to the BLS.

http://www.bls.gov/cps/cps_htgm.htm

> Employers think they are too young or too old,