I found a highly educational report in Radio-Insight. I have furnished a link so you can read about the various formats.

Yes, Chicago's WTMX-FM 101.9 made it into this article.

Classic Rock's survey goes back to 1964 with "House of the Rising Sun" by The Animals on New York's Q104-3 WAXQ-FM.

CHR with New York's Z100 WHTZ-FM, goes back to 1997 with "Wannabe" by Spice Girls.

Major market stations were sampled. I have little doubt smaller markets may skew older or younger.