I've been looking at Comic Book Ages, myself, on an on-and-off basis. What's interesting is that I don't see an explicity "Dark Age" like you do, but rather every age having its own version.
(I'll warn you now--this is long and probably rambling. My apologies, and I won't hold it against anybody for completely ignoring this message...)
I'll grant that years are inexact because they "sweep" through the industry at different rates, but something I've noticed is that, every so often, the "morality" of comics changes, there's a reboot of several key characters (Superman, Captain America, and Blue Beetle are the examples I see the most), and the aforementioned "darkness" midway through.
So my Golden Age runs from 1938 to 1954, when the Comics Code is established (with a mandatory moral code); in 1948, the Wertham has his symposium that results in book burnings and the creation of the ACMP. Superheroes don't go away, but they do need "patriarchs" (Superman or Batman, for example) to stay on the racks.
I see the Silver Age starting in 1953 (yes, I know), because Timely revives its big three superheroes and Superman sees the first of a long string of Krypton-oriented stories. The following year, the Blue Beetle returns, "Seduction of the Innocent" goes to press, and we get the Comics Code. Within another few years, we get the Martian Manhunter, the new Flash, the ANC collapse, and the campy sci-fi Batman. It's hard to spot the "dark" part of the Silver Age, but I might pin it on the rise of fandom, which pushes the books down a more directly commercial path (and sets up for the writers who are more fan than professional, later on down the line). Mileage may vary, though, and I realize that sounds insulting to a lot of people.
The lines I see are blurrier for the Bronze Age, because it could start as early as 1966, with major revamps to the Martian Manhunter, Blue Beetle (Ted Kord in 1967), and Wonder Woman. However, most of those were minor and for failing titles. In 1970, though, we see the Falcon joining Captain America, which started the arc redefining the character, and the non-Code Approved Green Lantern and Spider-Man issues (forcing a change to the Comics Code). 1971 brings us the Superman revamp, where his day job is finally updated, the silliest flotsam is abandoned, and Clark begins exploring his "place" in the DCU. And the DC Implosion puts the dark part of the age into place when financial concerns toppled a good chunk of the industry.
My Iron Age absolutely has to start in 1985-6. "Watchmen" and "The Dark Knight Returns" set the grim and gritty forces (our new moral cycle) in motion. "Who's Who" and Marvel's Handbooks make cataloguing a big deal even as the status quo is shattered. Captain America changes substantially in "Secret Wars." Superman gets an overhaul in "The Man of Steel." Blue Beetle even starts over as a result of "Crisis." Right on schedule, we get our collector bubble around 1991 and the horrifying garbage churned out by Image Comics (which itself changed the Comics Code, but nobody pays attention to it, by this time), leading to a lot of depressing and directionless garbage.
And I'd say that our current age started in 1999-2000. Geoff Johns turned the JSA into a "family" book, where your team and allies are more important than saving the world. That's carried over into every other book from all the companies, it seems. Marvel's Ultimate line showed up at the same time they abandoned the Comics Code entirely. There's no explicit change in Superman, but there was supposed to be (Waid, Morrison, and others put in a proposal, rejected because they were "too popular"), and a lot of those ideas showed up in Superman/Batman, Smallville, and Birthright in the following years. We get two new takes on Captain America. We have to wait a couple more years to get the new Blue Beetle, but there he is.
By my count, we're seeing the Age's decline now. The Siegels won their Superboy and Superman cases. Marvel killed off Captain America. Batman may or may not get killed. Countdown was utter nonsense and Secret Invasion doesn't sound much better. Final Crisis advertises other books, rather than telling a story. There's actually a pretty expansive list of missteps as paper costs are on the rise and nobody understands digital distribution.
In the meantime, I'm going to sit back and wait for 2014 or so. Steve Rogers is expected back as Captain America in 2012, I believe. Shuster's nephew will have his copyright case the following year, which should force significant changes to Superman. I assume someone'll revamp Blue Beetle at that point, too.
What's interesting is that my next cycle comes around when the Detective Comics copyrights will start expiring, meaning that we may see the first massive explosion of new characters to replace increasingly public domain Golden Agers. Or the companies may all go bankrupt trying to pretend that the Internet doesn't exist...
Hey, I told you it was long and rambling--I even left stuff out!