Irving Berlin, Cole Porter, and other songwriters of the Golden Era wrote popular songs that treated common topics clearly and simply. During the mid-1960s Bob Dylan, John Lennon, and Paul McCartney created a new kind of popular music that was personal and often obscure. This shift, which transformed popular music from an experimental into a conceptual art, produced a distinct change in the creative life cycles of songwriters. Golden Era songwriters were generally at their best during their 30s and 40s, whereas since the mid-'60s popular songwriters have consistently done their best work during their 20s. The revolution in popular music occurred at a time when young innovators were making similar transformations in other arts: Jean-Luc Godard and his fellow New Wave directors created a conceptual revolution in film in the early '60s, just as Andy Warhol and other Pop artists made painting a conceptual activity.
This is the abstract of a new NBER paper by David Galenson (gated NBER version), emphasis is mine. For the graph below, I used his Billboard and VH1 data for Golden Era and Rock songwriters respectively.