I have concluded that it is best to discard not only the names for past eras, but the whole concept. Following CS Lewis, I long ago discarded the notion that there was a Renaissance that followed the Medieval Era. Contemplating a recent description of Modernity - in order to contrast it with Post-Modernity - I decided that fascination with science goes back at least the 12th C, when it was called Natural Philosophy, and an insistence on black-and-white, either-or, true-or-false thinking goes back to at least the Church Councils that composed the Creeds. Thus, not a very helpful definition of "modern." In both cases I don't think I am cherry-picking isolated strains. These were the mindsets of learned men at the time. If it is argued that Modernity refers more to the less-exalted folk, the everyday people or even the peasants and what they believed, I will argue that they still don't think that very much. The idea is not entirely useless, but it obscures more than it reveals.
I am not simply arguing that the edges are fuzzy and subject to debate, nor that there were important strains of Era G that can be seen in Era F, Era E, and even way back to Era A. For each strain individually, say in music or in land rights around fuel in England, those can certainly be found and this has long been known, even by people who very much favor the idea of Eras. No one would consider those a disproof. My complaint is that while all aspects of a culture influence each other, they do not move at the same pace or according to vast overarching ideas. They muddle along, bumping into each other, and clever historians perceive patterns even when they aren't there. Because that's what human beings do. (I recently learned that the more intense version of this is pareidolia, seeing shapes in clouds, or faces in the shadows.)
I think this started with the thinkers and writers "who had the temerity to call themselves The Enlightenment,"as CS Lewis said. They divided previous history into Classical, Dark, Medieval, Renaissance, and their own age, inventing the idea of Renaissance because it fit nicely with their own ideas about progress, especial with regards to not paying any attention to the Church anymore. Not coincidentally, it made them look really smart. Next, Marx divided history into eras according to his ideas of progress and his prediction where it was all going. The difficulty is that feudalism turns out to be a less-than-helpful idea, another pattern that we imposed on events that don't really fit the definitions all that much - and capitalism, when you take definitions of any strictness, stretches back to 12thC Italy.
The Dark Ages weren't really dark, except in the sense that we don't have many written records from them. You can name something The Age of Exploration, but you can't put a beginning or an end to it. The discovery of the New World was indeed a turning point. Lots of people got rich, and sometimes different kinds than the ones who had gotten rich before. 90-95% of the inhabitants of two continents died, mostly from disease. Much has been written about how Europeans started to think differently, upon realising there were these other people out there, but I don't think you can show it from the record. Just the usual sort of gradual change we see in other times and places.
My own prejudice is that these are not the folks we should be taking our cues from.
The world may have been ready for such categorisation, though, as it took off and every historian started doing it. It took over the textbooks, and we structure our thinking around them now. Christian authors in the 19th C trying to explain the Revelation to John divided the Church into various ages which were represented by the churches John addresses. We like this stuff. We like making lists and lines and imposing patterns. We should stop doing that. It teaches us things that are not real.
I think such rough boundaries are justified along single lines. In arts we can have Romanticsm giving way to Neoclassicism followed by Historicism or whatever. But the usual picture we have of one generation rebelling against the previous artists neglects that there is considerable continuity, and plenty of composers or sculptors or playwrights who don't fit either category. They get neglected in the textbooks because they don't fit the narrative. But these strains do not all fit under an umbrella. There are changes in finance and changes in metalworking and changes in agriculture and changes in religion and everything else. They do influence each other, as I said, but not so much as advertised.
It can be argued that such categories are necessary to give some structure, or we could not learn anything at all. I myself have long said we need cuphooks to hang our cups on or they just fall into a jumble. Yet I am not sure this is true about history and culture and even less sure that this is the only structure anyway. We theoretically could discard the categories like training wheels once we have mastered riding. Except we don't, and it causes us to picture history falsely. Professional historians may be able to manage the discarding of eras. they say they can, I don't know if that's true. But for the rest of us who know only the outlines, we ended up having the outlines be the only things we know.
Who gave us these outlines, and why, we should ask.