I've always regretted that they had to call the reclusive author in Field of Dreams "Terrence Mann" instead of J.D. Salinger, though having him played by James Earl Jones instead of, say, Dennis Hopper, was what made that movie.
Anyway, I love this scene in particular, and always thought this was likely what J.D. Salinger would have said, too -- "Oh my God, you're from the 60s! Out, back to the 60s, back!"
Who knows, maybe he did actually say this to W.P. Kinsella.