Good question.
The Golden Age is generally accepted to have ended with
All-Star Comics #57 (cover date March 1951), the last Golden Age appearance of the JSA, and then the Silver Age beginning in
Showcase #4 (cover date October 1956), Barry Allen's first appearance, with the five years in between being either an untitled interstitial period or "the Atomic Age." That works fine for Flash, Green Lantern, Hawkman
et al, characters who were actually reinvented. But Superman, Batman, and Wonder Woman were all published continually from the Golden Age through the interstitial period/Atomic Age, on through into the Silver Age -- there is no definitive line of demarcation.
Wonder Woman is the easiest, because
Wonder Woman #98 (cover date May 1958) retold her origin, removing all of the WWII references, with some other minor changes (her mother went from a brunette to a blonde). In issue 99, a new origin for the "Diana Prince" identity was introduced, and in subsequent issues more changes were introduced. So for WW, the Golden Age stopping point is
Wonder Woman #97.
But Superman and Batman are a lot harder. Some would like the "yellow oval" intro to be Batman's Silver Age starting point, but that doesn't work because he'd already been appearing with the Justice League for a couple years
sans oval. For Superman, the best argument would be around
Action Comics #241 in June 1958 (first Fortress of Solitude) or 242 (first Brainiac & Kandor), or 252 (first Supergirl). But there really is no definitive answer, other than "sometime in the mid to late '50s."
(For a much, much more in-depth look at this,
click here.)