Rather than a cutoff date (1900 or 1960 or whatever), might we differentiate between "trad" and "composed" songs based upon whether they were written to be recorded and sold to a mass audience?

If not based simply on whether or not the writer is known at all, regarless of how ancient or noon-commercial...

By the way, "You Are My Sunshine" was written by Louisiana Governor Jimmie Davis, and was already quite popular before he sang it (incessantly) on the campaign trail; probably helped him a lot. Songwriting politicians are pretty rare, I would think.