The only one for which a precise beginning date can even be guessed at is Bluegrass. That came into existance on the date of the first public appearance by Earl Scruggs as a member of Bill Monroe's Bluegrass Boys. Monroe had all the other elements of bluegrass already in place; it only needed the Scruggs banjo style to "jell." It was in 1946, but I'm sorry, I don't know the precise date.

Peter.