There is one ingredient that seems to characterize folk music for me above all else. That is the aspect of accessiblity. In each sub-culture the music is available.
I subscribe to the communal theory of folk music. If a gathering of people (hopefully generational) can sing the song, chances are, it's a folk song. Granny will not necessarilly know the latest hip hop tune or a Beatles song. Some of the younger generation lamentably do not know a lot of the earlier folk songs such as Down In The Valley or You Are My Sunshine. I believe however that the latter songs are more accessible to people. The old country bromide about "three chords and the truth" are still applicable.

Frank