Getting back to this for a second, I think this gets at the point of what do people mean by "zero to hero"?
If we mean going from the absolute lowest possible spot on the power scale to the highest, generally speaking D&D has never assumed that. Even a first level fighter in 1974 OD&D was a "veteran", and started with 1+1 HD instead of 1HD like your generic soldier or orc, goblin or skeleton.
The 1E/2E crossover Greyhawk Adventures supplement included optional zero level rules for playing adventurers weaker than 1st level 1E starting characters, and earning your way to 1st level.
OTOH a metaphoric "zero to hero" is certainly a thing D&D has always done if by that we mean "start weak, get absurdly strong".
I agree with Willie that D&D has generally supported both the concept of first level adventurers being complete rookies with talent or veterans with at least some experience, but who are still not much tougher than an orc or goblin.
Well, the comment that sparked this thread was someone saying that they miss the
narrative of players starting off as inexperienced farmhands and whatnot. I want to point out that when I was playing early D&D and AD&D, nowhere did I ever see any indication that this was what was expected, but there's a lot of self-identified "old skoolers" who claim that's how the game was.
And that's what made me remember the picture of the old warrior in the CFH next to the Myrmidon Kit (the same picture is on the cover of
Fighter's Challenge) and I wondered if this was the first time the game came out and said "no, actually, you're not Cowhand #5". I'd forgotten about the old class titles, and "Veteran" is fairly telling.
I do think Zero to Hero gets conflated a lot- there's the narrative aspect, which apparently was never proposed by the books, but just sort of manifested out of the aether (probably by DM's thinking of heroes who are assistant pig keepers, farmboys, or lowly squires who go off and have adventures), and the "start off super weak and get more powerful from there", which has changed over time. In AD&D a 1st level character might be challenged by a thrown knife or a housecat. In 4e, they have enough hit points to face an ogre, and while 5e has scaled back from that, characters are much more likely to survive. In fact, once they get to level 2 or 3, it becomes almost impossible to die outright, even from a critical hit!
Either way, if the game did change the way some say it did, 2e apparently wasn't the moment where it happened. Thanks for the replies, everyone!