Interesting. Several inflection points mentioned in the threads summarized below.
Robin, Detective Comics #38 (cover dated April 1940)
Captain America Comics #1 (cover dated March 1941)
Pep Comics #22 (cover dated December 1941)
Wonder Woman, All Star Comics #8 Dec 1941/Jan 1942
Last 68 page Tech, Detective Comics #75 May 1943, Last 68 page Marvel Mystery Comics #44 June 1943
We could say that Robin was an appeal to broaden the market into younger readers, Cap might have been the epitome of the WWII patriotic superhero, Archie representative of the beginning of the "fall" of GA Superheros and Wonder Woman the epitome of the female superhero.
Given the feedback here, I think it's reasonable to move our imaginary "Peak Golden Age" date back 14 months from Oct 1940 (indeed a good year and month) to December 1941.
The "peaking" could be further evidenced by the page size drop (inflation expressed in a manner other than price increase) that occurs 18 months later.
Comic circulation peaked at 75 million per month around 1952 - the end of the Golden Age.