Hi,

productive careers of folk singers are mostly limited because of many reasons.
My answer to this thread might be, although not being comletely sure about this, that Pete Seeger is the one and only. He started in 1940 together with the Almanac Singers ( Woody Guthrie and others)and is active now for more than 60 years.

Any other ideas? Please concentrate on folk. Blues might be another cup of tea.