A Guy Called Tyketto wrote:

When did the Wake Turbulence advisory become standard in the
.65? From what I've read (I got into aviation in 2000), it happened
shortly after the King Air crash at KSNA that killed the founder of
In-N-Out Burger. It was concluded that the King Air had been caught in
the UAL B757's wake, rolled into a steep descent and crashed.

I don't know when it started, but might be able to narrow it a bit.

There were extensive tests done by NASA in the 1950s into the subject, and
various Advisory Circulars were issued warning pilots of the danger. One I
found that dates from early 1965 mentions that controllers might use the
expression "Caution, wake turbulence". It therefore goes back at least
that far.