I think the target demographic of AD&D was high school and up. 12 would have been a little young to be a target demographic, but Gary probably wouldn't turn away kids that young(and I think his kids played at those young ages), though he probably would have modified his games a bit and perhaps not included harlots in those games. He'd probably have that same expectation of DMs at large that ran games for younger kids.
I don't think OD&D and early AD&D had a target audience. It was more like a bunch of nerds discovering something fun and new, and wanting to share it with whoever was interested. Once bigger money got involved, and especially once a corporate vibe took over TSR, things like target audiences became more center stage.
The big difference now is that D&D is mainstream. Even up to and through 4E, it was still very niche - and the bulk of the player base was long-time players. So we're in strange waters, because D&D has gone mainstream - even more so than the 1980s, if only due to the changing nature of media, but also, I think, because "geek culture" is overall more mainstream. WotC (Hasbro) wants to maintain and grow, so they're particularly conscious (maybe overly so) to what is trending right now, in terms of socio-cultural sensibilities, fantasy aesthetics, trends, etc.
And they're also very cognizant of the fact that not only has the player base greatly expanded, but it is now much younger. Meaning, there was the first Boom in the early 80s when Gen Xers got into D&D, then in the 90s it was mostly Gen Xers continuing to play, and older Millenials coming on board. So there was still youth, but a large segment was now in their 20s and 30s. This continued into the 2000s, with the original "D&D Boomers" (Gen Xers) entering in their 30s and 40s, and the smaller "second wave" (Millenials) in their teens and 20s. What WotC discovered is that the younger Millenials--or first true Digital Natives--weren't jumping on board - the "third wave" wasn't really a wave but a ripple; it was mostly maintaining the remnants of the first and second waves. This is partially why they changed things up with 4E, trying to appeal to video games players.
Well, we knew what happened - not only did it not work--in terms of increasing the player base--but it fractured the player base. So WotC said, "better to have a bird in hand than two in the bush," and tried to get the band back together. Thus, 5E. And then something remarkable happened: it not only caught on with younger folks--younger Millenials and Zennials--but exploded.
So we have a new Boom, and the player base are mostly those new to the game. If you first played D&D in 2014 or later, chances are you are relatively young - a Zennial or, at least, a younger Millenial. So that's what they're looking at as their primary target audience: people who grew up with iPhones, for whom the Marvel Cinematic Universe looms larger than any other "geek franchise," although still augmented with stuff like Harry Potter, Lord of the Rings, and Game of Thrones.