John, for what it's worth, all the research I've seen would suggest older traditional singers not affected by Tin Pan Alley and the folk scene learnt the great majority of their songs in their youth.

Where ballads are not known to have been in print and are significantly different from British versions and fairly well spread this would indicate at least a couple of generations in oral tradition in America. Of course this does not apply so much to pieces like Barbara Allen which have been constantly in print and sheet music since the early 18th century.