Those retcons kind of confuse me too...I'm not sure if the "Spear of Destiny" had anything to do with it, though I do remember that the late 70s had stories about their post War retirement after appearing before Congress...way after my comics time.
As I recall, the Spear of Destiny was a Roy Thomas idea, which explained why some of the all-powerful heroes like Alan Scott or Earth-2 Superman didn't just go to Fortress Europe and end World War II in one stroke. Hitler had the Spear of Destiny, and wielded it, and so Europe could not be entered or invaded by superheroes.
I DO remember thinking that Black Canary looked pretty smokin' when she came to Earth 1, I never thought she was pushing 50...
Hehe, I agree. As good an artist as he was, Sekowsky's pretty women were never as neat looking as his weird, muppet-y monsters and aliens. Black Canary really entered into va-va-voom territory with Dick Dillin around 1968 or so, which was the same time that she came over to Earth-1.
By that same token, I always like how effort was expended to make the JSA and Earth-2 heroes have a consistent timeline. A real problem with many superheroes is that their pasts roll up behind them. If their pasts roll up behind them, that means that events in stories are totally recontextualized.
An example would be for instance, the Englehart CAPTAIN AMERICA, which featured Richard Nixon as the leader of the Secret Empire. The whole thing had the air of "1974" about it. It would NOT be the same story at all if it was set in say, the early 1990s. Ditto for the Fantastic Four; they were a product of the 1960s space race, fought Russian enemies, and so on. If they left in 1993, their whole motivation for their first spaceflight
Basically what I'm saying is that the whole sliding timescale was a bad idea, and it was pretty gutsy of Roy Thomas and others to see the JSA as a "period" team shaped by the times they emerged, and to leave them there so they keep on making sense. More likely, the reason Roy the Boy did it was because of "Greatest Generation" nostalgia, but he did the right thing for the wrong reason.
This is why it is possible to really appreciate explanation for why heroes were active for long periods of time, such as the JSA's various youthenings, the fact Captain America spent his time cryogenically frozen in a block of ice, or Nick Fury's Infinity Formula.