Kottke highlighs this Businessweek article:
In many ways, this is an idea whose time has come, which is another way of saying that hip-hop, and its first-wave fans, are, well, old. Dre will be 50 in February; Ice-T is just 10 years away from his first Social Security check. Licensed to Ill topped the Billboard charts in 1987; three years later, hip-hop made up one-third of the Hot 100. By 1999, it was the country’s best-selling genre, with more than 81 million albums sold. The fans who propelled the early boom probably don’t know Young Thug from Rich Homie Quan, and don’t want to.
The obvious parallel is to classic rock radio — a format that emerged in the early-1980s as baby boomers rejected punk and disco, and radio execs realized it was easier to serve up old songs than convince their aging audiences to try new music. It eventually morphed into a touchstone of middle-age: Every so often, a cultural observer wakes up, checks his bald spot and wonders how Green Day or Smashing Pumpkins or some other band of his own youth got lumped in with Led Zeppelin and Aerosmith on the radio dial.
The BW article links to this 1986 NYT article on classic rock:
To hold on to older listeners, FM rock radio – once the primary outlet for new rock songs and performers -is cutting down on current music. Across the United States, so-called album-oriented rock (AOR) radio stations are playing an increasing amount of older material, sometimes as high as 80 percent. In some cities, these album-rock stations now face direct competition from a new kind of oldies format -”classic rock,” which draws exclusively on the 1960’s and 1970’s staples of album rock.