Amos, good point about the generations test. The other elements would include adaptation by specific sub-cultures over a period of time. Schubert songs have become folk songs in Germany through change. Show tunes have also. "Old Dan Tucker" is a case in point. As far as popular music, Stephen Foster's "Angelina Baker" became "Angeline the Baker", a mountain fiddle tune. Church music? All the time. "Uncloudy Day" as sung by the Carter Family for example.Or the music of Sacred Harp from which folk songs such as "Poor Wayfaring Stranger" or "On Jordan's Stormy Banks I Stand" are spawned. The generational aspect must be accompanied by the sub-cultural adaptations by specific folk cultures which engender as the folksong scholars say, "song variants".
Rap music is still up for grabs. It hasn't bridged a generation yet although it contains elements found in African-American music such as "Jivin'", "Playing the dozens", chants, street and jump-rope rhymes and blues hollers from earlier times as well as the legacy of the African Griot. Still, it's still a manufactured Hip Hop culture by record industry people and pop wannabees. There are no octengenarian rap singers as far as I know.
The generatinal test is still there but includes these other elements to make it valid, adaptations to different situations by specific sub-cultural groups which span generations and are identifiable as such.