Well, the first GA books I ever encountered were a small pile my uncle left behind, when I was about 5 or 6. They included a BLACKHAWK, an ACTION, an ish of CANNONBALL, one of JOKER (with Powerhouse Pepper in it), a BIG SHOT, and a LOONEY TUNES. By the time I got back to check on those things years later, they'd given them away. But my interest was really piqued early on by JLA #21-22, the first issues I bought, the ones with the Justice Society's first crossover. Also, Marvel put a faux Captain America in STRANGE TALES about that time as a tryout. Learning that there had been heroes before the present era was always a curiosity point, and I never stopped wondering about them or picking up information on them where I could.
Decades later, I had a batch of Fawcett CAPTAIN MARVELs and related books that I'd scanned and was looking for gap-fillers. I happened on this site. Mirabile dictu! The rest is historic.