As a jazz/blues man, I hesitate to put my head above the parapet and offend anyone (you know what a shrinking violet I am) but I'll add two bits of BS: My beloved Louis Armstrong when asked "What is jazz" said "If you need to ask you'll never know" (or words to that effect).I'm sure we all know what is folk music to US.
My lyrics folders are divided thus: Blues;Oldies but goodies (jazz, music hall, comic songs);Rock 'n' Roll and Rock 'n' roll love songs (1950s era); and Folkish which includes skiffle, campfire, Scots Irish, Welsh,parodies, Kingston Trio, Spinners, Corries McColl, Seeger songs etc. They may not all qualify as oral tradition but at least ape it.
As I mentioned in the Tavern thread a new BBC radio series on 100 years of Folk Music takes 1899 as the starting point when Cecil Sharp began collecting. Presumably before then people just had "Music".
When people at a party, pub, or campfire start singing Mozart or Beethoven, we'll know that's folk music!
But then, what do I know?