Is this working age population or all ages ?

That seems to be everyone above 16 years of age. It excludes inmates, that is penal and mental institutions (which in the land of the free is surprisingly sizeable chunk). Also excludes active military personnel. Notably it includes people who are disabled but are unable to work.

It is a historical range of working age, so it includes people who are 16 and over and everyone until the die of old age.