That's a big question, to which you won't get a specific answer; it depends on what you mean by traditional, for one thing. I don't know if anyone has done statistical work of that sort, though I think there's some concentrating on the repertoires of individual singers. Unpublished doctoral thesis territory in the main, I suspect.

That said, a large number (perhaps most) of songs found in oral currency and treated as "folksong" do seem to be no older than the first half of the 19th century or even later. Many can now be traced to known writers. Although there are certainly songs which seem to have had an independent life before their appearance in 19th century cheap print, my guess would be that they are fewer than used to be thought.

For obvious reasons, fewer songs survive from the 18th century, still fewer from the 17th. Very few indeed from before that, though there are some of course. As a general rule, folk songs are rather less old than people would like them to be, though we're learning new things about them all the time. Just recently, Steve Gardham mentioned that Here's to You Tom Brown, which Frank Kidson had thought 19th century, is a condensation of a 17th century broadside song.