The issue isn't how old is the *story* of "King Orfeo," or what they might have done with it in the Middle Ages. The issue is how old is the British *ballad*.

Not for me. The issue will always be the cultural continuity that makes us receptive to such things in the first place. This is how the ballad came about - with its vivid imagery & bilingual verse structure. As such I'd say it's essentially timeless in terms of mythic archetype and dreaming in which we all share.