To me it was not a matter of the Bronze Age starting as it was the Silver Age ending. The passing of the torch from Stan Lee to Roy Thomas, Marv Wolfman, Gerry Conway and the rest of the next Generation of writers formed the Start of the Bronze Age. To me Captain America 100 is Silver whereas Cap 110 with Steranko is Bronze. The Price increases from .12 to .15 to .20 are good benchmarks, and by the .25 giant books the bronze age is in total effect. So,to me, the early Steranko books, along with Neal Adams work on the Xmen are the prelude to the dawn of the Bronze Age.