Recently, I was reading through some issues of Superman and Action Comics from 1953 and 1954. These were books that I had never read before now, since they predated my earliest comics collecting years and were old enough that none of the people with whom I traded comic (nor the stores where I bought used comics) had books from that time period.
As I read through these books, I recognized several of the stories. "A Doghouse for Superman," a 1953 story from Superman #84 in which he was captured by aliens who treated him like a pet, was instantly recognizable to me because it was reprinted in Superman Annual #4 in 1961. Same for "The Outlaws from Krypton," originally published in 1954's Action #194 and reprinted in that same annual. "The Thing from 40,000 AD," which first appeared in 1954's Superman #87, was reprinted in Superman Annual #2.
What's significant about this is that none of these stories seemed at all out of place in a 1960 or 1961 comic book--nor would they have seemed out of place in a Superman comic from 1965, or 1968, or any issue from the Mort Weisinger-edited era of Superman (which lasted until 1971, when Julius Schwartz took over the Superman titles). By 1953, the Silver Age Superman pattern was already established; it would continue largely unchanged for almost two decades.
That's pretty amazing, when you think about it. Look at today's comics and see how frequently characters undergo major reinventions, revitalizations, relaunches, recasting, and reinterpretations; the idea of a character with a two-decade status quo is all but unheard of. Certainly, Jerry Siegel and Edmond Hamilton and others expanded on the Superman mythos in the late 50s and early 60s, but not in such a way that the earlier stories seemed like they would be out of place.
So when did the change occur? When did the "modern" era of Superman begin--a period that might mark the beginning of the Silver Age for Superman family titles, at least? My first thought was Superman #76, featuring the first Superman-Batman team-up, but I did some more reading and found that at least two issues before that have a decidedly "modern era" feel to them. So I read through Superman #s 60 through 83, just to see when the tone of the books took on a decidedly modern "Silver Age" feel.
The transition for Superman seems to take place between Superman #73 and Superman #74. Superman #73, the November/December 1951 issue, has a decidedly Golden Age look and feel as Superman confronts circus strongman Hank Garvin, who stands his ground against a steamroller. But Superman #74, the January/February 1952 issue, pits Superman against Lex Luthor, who has come into possession of Kryptonian weaponry and is using "The Lost Secrets of Krypton" against the Man of Steel. Had I read this story ten years later, it would have fit right into the Silver Age Superman canon.
I'm not sure what inspired the change, but as far as I'm concerned, the Silver Age doesn't start with Superman #76 or Detective #225 or Showcase #4. but with Superman #74. For Superman, at least, this is when the tone of the stories and the continuity elements of the Silver Age really come into focus. I may never know why the change occurred, but at least I know when.