The first recorded use of "disc jockey" was in Variety on August 13, 1941. Radio was about twenty years old at that point depending on when you want to start counting. The term had to already be in usage from this context. But it's nice to put the date firmly in the golden age of radio.
"... Gilbert is a disc jockey, who sings with his records." The same issue also wrote "Art Green disc-jockeys from Manhattan Beach..."
Can anyone supply the full text? Or at least better attribute the quote?