Although it's quite old now, this post from James Maliszewski's blog Grognardia makes an attempt to categorize D&D into "ages" and James' attempt is strongly rooted in Hesiod's own Ages of Mankind, i.e., it posits a past Golden age followed by years of gradual decline, punctuated by events or developments that can be seen as significant enough changes to mark the boundaries of each age.
Of course, his perspective comes as a person who's enamored with the older versions of D&D and the "primary sources" that informed it, and he approaches those conventions and tropes with nearly religious fervor, seeing any deviations from them as heresies to be strictly avoided. That doesn't match my own experience with the game at all, nor--do I imagine--is it really a sufficiently common viewpoint that his ages will ever mean anything outside of the relatively narrow OSR community.
A guy in the comments section, on the other hand, takes a stab at constructing an alternative loosely based on the ages of archeology. I like this one better, as it posits, at least in broad strokes, a generally progressive history of the game. Neither one really maps perfectly to D&D, but I believe the archeological model is at least a better starting place than Hesiod (which was also mimicked by comic book fandom, which many gamers also belong to.) But unlike in the archeological model, I don't see a progression of uninterrupted progress; there is a decadant Dark Age period as well.
Anyway, rather than merely critique Mr. Malizsewski's model, I'll offer up my own alternative to it; one that, like his, is firmly rooted in the personal taste and subjective quirks of its author, but one which I think is more broadly applicable outside of the OSR.
1. The Paleolithic, or Old Stone Age (1974-1977) Comprising the period during which OD&D was published up until AD&D and BD&D were published in 1977. During this age, the rules were often ad hoc, made with little reference to other rules, arbitrary, and generally insufficient (in fact, they occasionally made reference to competitive products, assuming players would be familiar with and own them) and assumed a wargaming advent into the hobby. With very little direction and insufficient rules, gamemasters, or referees as they were often still called at this stage, had to do an awful lot of interpretation to cover missing spots, and there was little in the way of tools to support them.
2. The Neolithic, or New Stone Age (1977-1984) As with the archeological age, the Neolithic is characterized by a "revolution" in technological competence; the OD&D game, while not officiall retired right away, was largely replaced by two differing lines during this age, the game eventually known as AD&D and the game that later came to be called BD&D and its eventual successor, the Rules Cyclopedia. This is also the period of D&D's greatest mainstream popularity, including a cartoon series and a toyline, and the presence of the rulebooks in department store catalogs and shelves. While the game still had a lot of arbitrariness and is still pretty clunky and counter-intuitive in many instances, conditions inherited from its predesessor, both AD&D and BD&D made a concerted effort to better organize, write, and present the rules of the game, adding rules to cover more circumstances and otherwise making a game that ran relatively well compared to what came before.
3. The Bronze Age, (1985-1991) Shortly after the publication of the mixed reviews book Unearthed Arcana, Gary Gygax was finally completely ousted by the Blumes and D&D went into the custodianship of other hands than its creator. One of the changes that came about around this same time--perhaps able to be ushered in with the departure of Gygax--was the advent of Dragonlance, and the fallout of that hugely successful enterprise. The first Dragonlance novel was actually published in very late 1984. It wasn't the first novel associated with the D&D game, but none before then had really been very successful, and TSR were initially sceptical, ordering the minimum print run (50,000) and wishing they could have gone even lower. As it turned out, the Dragonlance novels were immensely popular, and paved the way for TSR (and later WotC) becoming one of the most prolific and well-known publishers of fantasy literature. A few months before the novel came out, however, the first Dragonlance module came out. Following on the heels of another huge successful Tracey Hickman module, Castle Ravenloft, this is perhaps more important for its impact on the game itself--the realization that a large portion of the audience wanted some setting and some story and interesting characters to their D&D, and the days of modules that were just random encounters with disposible characters and monsters in nonsensical dungeons was truly over. This focus on story continued well into the next age (and beyond--it's still a potent force in the industry).
4. The Dark Age, (1992-1999) By this time, AD&D had had another edition published, and the BD&D line was consolidated into the Rules Cyclopedia but those changes in the rules were not as important as three other changes that happened during the "Dark Age" that profoundly impacted D&D. The first was the mismanagement of the company financially by its owners. A number of really bizarre products were offered and flopped (Dragonlance was moved to a new system not D&D, for instance--failed card games and dice games also proliferated) and a general perception that TSR was putting out a number of shoddy products quickly to attempt to shore up bleeding balance sheets was widespread. When this got to its worst point, TSR, completely unable to continue paying its bills, essentially shut down completely and the product pipeline, flawed though it was, was shut off entirely until Wizards of the Coast came and acquired them, paid off their debts, and rationalized their continued product line, as well as starting development for a brand new edition that would make a much cleaner break with the past than the prior one had (more on that next age.) Besides on the business side of the house, however, two other important things happened to D&D. 1992 was--more or less--the beginning of a number of years in which TSR put out campaign setting after campaign setting. While this is frequently cited as a "bad thing" from a business perspective, at the same time, this is also seen as a Golden Age for setting aficionados, of which there are a lot amongst gamers, and many of these settings are remembered quite fondly. The third thing that happened to D&D was external--in 1991, White Wolf published the first of its Storytelling line of RPGs: Vampire: The Masquerade which, for pretty much the first time ever, was seen as a serious and significant competitor to D&D. This had a side effect of bringing a bunch of new players into the hobby, as well as a whole new conversation about how games should or could at least be played.
5. The Iron Age, (2000-2007) With the release of Third Edition Dungeons & Dragons (and the final collapse of the AD&D and D&D split) we get ushered into the Iron Age, an age of technological advancement unheard of in D&D itself, at least, for a long time. Not so much because of third edition itself (although as a cleaned up and rationalized bit of game design, it finally removed a lot of the arbitrariness and weirdness that had plagued the rules up to this point) but because of the Open Gaming License, or OGL. See, Peter Adkinson, Ryan Dancey and the others from Wizards who bought TSR did it specifically to rescue D&D because of the terrible mismanagement it had suffered during the Dark Age and the risk that it was going to--literally--disappear: be locked up in bankruptcy court and out of print indefinately. Since saving D&D was one of the primary decisions of the Wizards management--who did it as fans of the game, not necessarily as businesspeople (although making the business profitable was certainly a high priority goal), they wanted to come up with a way to protect D&D--give it to the fans forever--so it could never be mismanaged to the point where it was foreseeable that the game itself would disappear. The OGL was the vehicle that accomplished that; by putting most of the rules into an open, nontransferable and nonrevokable license so that anyone could use it, the third edition of D&D was immediately insulated from any decisions that the owners of the game could ever do to it. A remarkable (although not unforeseen) side effect was that a lot of great stuff was issued by third party publishers, greatly enriching the game beyond what it ever could have been able to do even at the height of the "Neolithic Age" when it was at its peak popularity, or even during the decadent product glut of the Dark Age. Of course, a few things happened during this Iron Age which were not foreseen. The most important one was that WotC was sold by its owners to Hasbro. This was primarily to bail the principles of WotC out of personal debt. It was also widely believed, and the WotC management had been led to believe as much from the Hasbro guys, that since WotC was more profitable than Hasbro, that it would continue to operate as it had, and that in fact the WotC management might be groomed for bigger positions within Hasbro to restructure that organization to match the WotC one. This ended up not being the case, and D&D shortly started showing the effects of Hasbro management, including a production schedule that was more focused on short term profits than long term health. Another unforeseen--or at least not committed in writing by anyone that I ever saw--circumstance was that third party publishers gradually stopped issuing very much stuff that supported the OGL, and instead used the OGL to create their own variations on the rules which were no longer compatible with the main game and therefore competed with it rather than supported it.
6. The Splintered Age, (2008-Present) Finally, in 2007 Wizards announced that the third edition (and it's revision, 3.5) had run their course and an all new design from the ground up called Fourth Edition (4e) was going to be coming out in 2008. Furthermore, it gradually came to be apparent that Wizards was not going to support the game with the OGL, but rather with a much more restrictive GSL that was not popular with the remaining significant third party publishers. The new edition came out, and sure enough, it only superficially resembled D&D that came before, which greatly divided the fanbase. In the Splintered Age, D&D was less defined as "the game that bears the brand name of Dungeons & Dragons" and is rather a state of mind that can be applied to several games. The important ones, aside from 4e, include Pathfinder, published by big time third party publisher Paizo, which takes the old OGL and updates and fixes some percieved problems with it. Pathfinder, while not bearing the D&D name, is seen by many fans as a more direct and "legitimate" heir to the D&D legacy than D&D 4e, and although sales numbers are notoriously hard to pin down, common wisdom and some circumstantial evidence suggests that Pathfinder sells in numbers comparable to D&D itself. In addition to Pathfinder, out of print 3.5 games appear to still be fairly commonplace, and on the used market, demand (and therefore price) for 3.5 books remains high. I know my group still plays as such. The third significant alternative is the OSR--or old school rennaissance, which sprung up independently from--although at about the same time as--the announcement of 4e. The big enabler of the OSR was the release of OSRIC--a fan creation that used the OGL to "back-engineer" a document that was compatible with 1e AD&D. When it was seen that this was going to pass any potential legal challenges hurled at it, the floodgates opened, and a whole movement--a do-it-yourself movement with a lot of small heads--sprang up around playing D&D using "retroclones" like OSRIC, including Labyrinth Lord, Sword & Wizardry and others, or using OGL compatible games that hybridized old school versions of D&D with some more modern innovations in game design, like Castles & Crusades and others.
This is where we are today, and although rumors of a 5th edition of D&D being in the works are fairly fast and furious and circumstantially supportable, and not denied by WotC, it seems unlikely that the splintering that occured within D&D will be undone anytime soon--the OSR crowd in particular pays little attention to what's going on at WotC, and the Pathfinder crowd is made up in large part of people who actively rejected 4e and WotC completely and may be very unwilling to give them another look.