Q:A: Dividing the history of comedian books into a series of ages is a pretty easy thing to do,but picking out one single issue that serves as the hard, immutable dividing line can be a tricky proposition. Showcase #4, and the first appearance of Barry Allen as the Flash,is the one that everyone seems to hold always agreed on as the "Official Start Date of the Silver Age," and it's approximately as good a dividing line as you're likely to find external of Action Comics #1.
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Source: comicsalliance.com