Pre-any kind of “Crisis”, when the trope of a multiverse with Earths-One, Two, Three, etc. was firmly established:. At what point in the runs of the titles “Superman”, “Batman” and “Wonder Woman” do the stories stop taking place on Earth-Two, and begin taking place on Earth-One?
For “Flash”, “Green Lantern”, “Hawkman”, etc. there’s a clear demarcation. The original series were discontinued one by one. When the revamped titles began, they featured all-new origin stories and backgrounds for these characters. But Supes, Bats, Robin & Wonder Woman (not to mention a few also-rans such as Green Arrow & Aquaman) kept right on chugging from the late 30s/ early 40s with no breaks. Did the Powers That Be at DC Comics ever set a definite issue # for any of the series in which the swith from ‘golden age’ versions of the characters to ‘silver age’ versions took place?
There are comics blogs who dedicated themselves to reasoned speculation on this subject. Hell, I’ve even seen a detailed analysis of the same question for Aquaman. (He and Green Arrow are the other two continually published Golden Agers that survived to the Silver Age.)
The characters that survived the Golden Age - that also included Green Arrow and Aquaman - changed gradually between the Golden and Silver Ages, so at which point they can be considered different characters isn’t clear.
The Earth-2 characters shouldn’t, strictly, be called the Golden Age characters. They were based on them, of course, but there is a lot of Golden Age material for each of the characters that wasn’t in-canon for the Earth-2 characters any more than it was for the Earth-1 versions. (Batman, the gun-toting killer, for instance.)
The first adventure of Superboy, who was only part of the Earth-1 version of Superman’s history, was published in 1949. Another thing to muddy the question.