I don't disagree with the general premise that it's easier to identify books by the date they were published, but ages generally convey a particular feeling, A book from 1990 feels a lot more like a book published in 1989 from the same company than it does like one published in 1999. I also agree with what you pointed out about the two issues of JiM. They belong to the same age because they have the same feel, regardless of whether Showcase 4 was published between them. Different titles enter the new age at different times, as people catch up with the trendsetters. The Silver Age was largely a revival of super-hero comics, after the Golden Age diversified into all sorts of genres (westerns, romance, science fiction, etc.). The Bronze Age was largely defined by pushing the boundaries of the content and testing the limits of the CCA (initially, anyway, and then generally books became more mature). I think the Copper Age was largely about a few things, with creator rights and changing distribution systems at the core. It's a little earlier than most people suggest, but I think the catalyst for the Copper Age was the DC implosion. That led to many departures from DC, the most important amongst them probably being Archie Goodwin and Larry Hama. They both ended up at Marvel, where Goodwin helmed the launch of Marvel's first (mostly) creator-owned imprint (Epic), and Larry Hama's work on G.I. Joe led to an explosion in licensed material.
I see the formation of Image as being a relative midpoint of the Copper Age. Sure, the fight over creator rights culminated here, but the first few years showed a lot of the problems with having a line that was run by the creators, with late and canceled books a regular occurrence. I don't think it was until Liefeld and Lee came back to Marvel for a bit that Image started settling down into what it more closely resembles today.
It's interesting that you point out our current situation as the potential beginning of a new era, especially as Bad Idea is about to test new distribution methods. Eniac #1 might be the first book of the new age.