yeah, we seem to get '60s once-a-month; but we get early-'70s regularly. Odd.
Beginning of the Rock Era (as opposed to rock 'n' roll)?
I'd say that Rock (as opposed to Rock 'n' Roll) began somewhere around 1965, and possibly
even 1964. Titles are not exactly springing to mind right now, although Dylan's Blonde On Blonde
is a prominent example, but likely not the first. Someone else will probably provide the obvious
answer, and I can do a facepalm when I read it.
That makes sense, but I was coming from the perspective of a radio pro. Looking at the chart from the article you posted, "Why Classic Rock Isn't What It Used To Be," Sixties songs make up roughly 11% of total classic rock airplay, with '66 and previous combining for less than 2%. Data like that could lead an industry professional to conclude that the mid-'60s are "over" and redefine the Rock Era as '67 and after, beginning in earnest in the Seventies.
I'm not sure what your point is, but it seems like there's a bit of circular logic involved. It's the
"industry pro" who is responsible for the fact that '60s songs comprise 11% of classic rock airplay,
etc. It's probably based on some kind of survey, but that quickly devolves in its own loop. (Songs
don't get played because they don't have recognition, and they don't get recognition because
they don't get played.) Radio professionals may redefine their version of classic rock, but that
doesn't change when rock 'n' roll became rock.