Folksongs and folktales can only be reliably dated to the first known recorded version, though obviously they are often considerably older than that, and a tentative earlier date may sometimes be arrived at from things the source says, such as "I learned this in (date)" or "from (identifiable person)"; the point is that, beyond the available historical evidence, all else is just guesswork, sometimes educated, all too often sheer fantasy.  That doesn't mean that there aren't songs or stories still (or recently) current that are very old indeed; just that it's usually impossible to prove.

Malcolm