Back in the day, fans hardly ever saw performers in sports jerseys. Aerosmith never went on stage with Red Sox or Celtics garb, the Motown groups didn't represent the Tigers or the Red Wings, and I don't think the New York City punk bands ever sported Knicks, Giants, Jets, Mets, or Yankees attire.
Somewhere along the line, sports jerseys hit the stage.
Here are the earliest examples I've found of performers wearing jerseys:
1982: .38 Special drummer Gary Moffatt represents the Atlanta Braves and pitcher Joe Cowley in the video "Caught Up In You" (1:45 mark):
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1984: Spinal Tap's legendary guitarist Nigel Tufnel wears a Sadaharu Oh jersey while on stage in Japan (see 2:40 mark):
These are the oldest two examples I could find. I know there has to be more. Maybe Run DMC, the Beastie Boys, or other old school rap legends represented their team during their heyday.
Consider this The Serious Tip's first bleg. When did musicians, rappers, or other performers start wearing sports jerseys?
If you know of anything pre-1982, please include a link. Thanks.