I'm new here, so late to this thread, but wanted to add my .02.
How about Copper age starting around June 1982 with the first Marvel Limited Series "Marvel Super Hero Contest of Champions". It was the first of it's type with a limited series that had nearly all of the Marvel Universe. This went on to bring about Secret Wars and Crisis on Infinite Earths limited series and now the almost annual limited series. And the copper era could end around the time Valient did their Unity series that caused so much grief, which was the end or 1992. That's an 10.5 year span that really did shape the way we see comics today, grandiose limited series that featured limited variants and bagged editions etc. Also the Independants appeared hot this period and you could also argue that it ended around the end of 1993 with the closing of one of the biggest Independants of this era, Eclipse. In fact, that's almost 14 years (actually 11.5) the start of the limited series with Marvel and the end with the closing of Eclipse (that was there during the whole time). Modern age begins 1994???