The oldest issue of Superman still in my possession that I remember reading as a kid was this one:
However, the cover date is April 1970, which would have made me 5 years old! I'm pretty sure I didn't really get into comics for another two years...my most powerful memories are from late 72 and all of 73. In retrospect I think I probably got this comic from an older kid who was always eager to trade me "vintage" DC's for the Marvels he preferred. However I distinctly remember getting my grandma to buy me this issue of Jimmy Olsen in 1971, because the cover blew me away. (To this day, it and Batman #251 are the most unforgettable covers I ever saw):
Back to Superman, I know that starting with this issue I hardly ever missed a month:
And I also remember finding an issue on a spindle rack with a cover that included Super-everything...Superman, Supergirl, Krypto, Comet, Super-Monkey...I thought it was the most terrific thing I ever saw. But I've never seen it again in years of searching and by now my memory's so fuzzy I dont' know I'd recognize it if I did see it.