1977, when the first book aimed at standardising forms and systems for all campaigns appears.
AD&D and its linear descendants (including the one about to be born) share the name, and
a lot of the tropes. I've enjoyed (and still do) playing them.
The Basic/Expert line and Mentzer's and the Rules Compendium also share the name and
a lot of the tropes. I've enjoyed (and still do) playing them.
But, as dialgo is so fond of saying, they aren't what D&D was, either.
Which is perfectly alright.
Now, an argument can be made that D&D stopped being D&D with the publication of
Greyhawk, the first supplement (and the rest that followed), but the rules and guidelines suggested therein were less explicit about being the standard form by which the game ought to be played.
The moment we became an audience more interested in what designers were doing with the rules than where we could take it on our own, D&D was no longer D&D.
So, AD&D is D&D with Gary's house rules, Second Edition is Zeb Cook's and Steve Winter's, Basic is D&D filtred through Eric Holmes' eyes, or Tom Moldvay's, or Frank Mentzer's, Third Edition is how Monte Cook and Jon Tweet and Skip Williams chose to run with the ball, and Fourth will be the result of how Mike Mearls, Andy Collins, Richard Baker, and others choose to interpret things. Everybody's got a different take, and none of them are necessarily bad, or better, or worse.
But none of them is D&D the way I see it, all the time, either. D&D used to be about pointing you in some general direction and saying "there, go to it".
There're a lot of reasons why that stopped. None of them are evil. IP protection issues, general enthusiasm, companies (with peoples' livelihoods attached to them) needing to see expanding profit margins in order to continue, players and Dungeon Masters who were harried for time or who just outright
liked what designers were publishing are some of those reasons, and as I said, none of these is a bad reason.
But, as I see it, when the game started to become largely standardised overall, it stopped being D&D and instead became "x's" D&D, which is quite similar in a lot of respects, but not enough to be entirely the same.
So, with a

to show it's not Gospel, or even true, just my opinion, I'll let it go at that.