As long as people who think that they are 'folk', also feel free to use any subjective definition they choose, I'm afraid it's hopeless.

To ME it is stuff that is generally older and which got transmitted thru more than several generations and often 'processed' as it was handed down.

To others, it can mean no more than "what I remember from my formative years and is now subjected to 'best 100' lists on some web site."