John, I believe you posted the wrong question right at the start of this thread. YOu are posing an Irrelevant When.

Don't ask When a song gets to be folk, ask How. I would say that a song gets to be folk:

1. If it has a melody people can easily remember.

2. If it has repetition, such as a verse/refrain form

3. If it uses simple chords. Am, not A7aug6dim

4. If it deals with timeless events. Love, birth, death, beer.
Not sportscars, texting or computers.
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Once I was having a spat with the DH about another relative. I said, "Your sister said he only calls when he wants money," and the DH huffed himself up and demanded,

"WHEN did she say that?" Right then I realized that when she said it didn't matter a bit. That's when I conceived the idea of the Irrelevant When.