For Marvel in particular the 20¢ covers were a departure from the previous look - though I would include thier one-month 25¢ covers as the start of the new look - picture frame covers to be precise. At the time while I wasn't thinking that a new "Age" had begun, the 20¢ era cemented the idea that comics were changing - Kirby was at DC, "Weird" and "Horror" titles were popular, Marvel was cranking out more and more reprint titles, and many of the "relevant" and "hip" books and storylines begun in the 68-70 period were either done with or coming to a close; GL/GA, Hawk and Dove, Captain America's "easy rider" phase, Nick Fury, agent of SHIELD, stories with"Campus" in the title, drug stories, the "mod" Wonder Woman, etc.
I think of the Bronze Age as when more 1970s pop-culture trends began to be reflected in comics like Kung-Fu and Blaxploitation movies.
While many of the contenders for first BA book have 15¢ covers - by and large the look of comics didn't really change right away when the price jumped up from 12¢.