I think that the majority of songs that most American would acknowledge as "folk" songs were newly composed not so very long ago. "Goodnight Irene", "Blowing in the wind", "This land is my land", any John Denver hit, are commonly sung around campfires and described as "folk", and they're relatively recent and easily traced, compared to something like "Yankee Doodle" or "Barbara Ellen". Spaw's ire notwithstanding, the reason Amos' question comes up and people write long tomes about it is that there's no simple answer. Fertile ground for Academics -- they can pontificate forever, and squabble among themselves, and write infinite numbers of PhD theses, while the rest of us have a good time singing.