It is possible for a tradition to have a known starting point, and still be a tradition.
In Tudor times brides wore green, if they had a change of clothes at all. Now the traditional bridal gown is white.
The traditional Christmas with turkey and all the trimmings and a Christmas tree is a Victorian invention.
Even more recently, you will often see yellow ribbons tied round a tree, marking an impromptu shrine to a road accident victim.
This custom was unknown in the UK until the song "Tie a Yellow Ribbon" was released in the 1970's, creating a new tradition.
The PRS definition of "Trad=No writer, no royalties" is fine for copyright disputes, but the traditions and and customs of the people of a country or a region are a much more fluid thing- and they do change with the times.
Perhaps it is the same with Folk. I think so.
Cheers
Dave