Two big problems I see with Eras or Ages:
1. The game (and several other games using the concept) relates the Age/Era to Game Progress. Civ changes the actual years/turn with each Age, which means the Entire Game World has to move from Age to Age in lockstep or the entire chronology of the game breaks down.
2. Some of the Ages are defined by specific , usually technological, triggers. Others, though, are Relative: the 'middle ages' fell between the Roman Imperial Golden Age (or at least, highly polished Brass) and the Renaissance. The Renaissance, on the other hand, was a Rebirth after a 'dark age'. Both of these require a specific sequence of previous events to be meaningful, and that sequence includes a loss of progress that is absolutely unacceptable given the game's relentless pursuit of Progress towards a defined set of Victory Conditions. Even if the prior coditions could be defined and programmed into the game, no competitive gamer would allow them to happen and if they happened, not restart immediately. Bluntly, this would be Unacceptable as a Basic Design for a competitive game, IMHO.
Even then. the reckoning date of each Age isn't really something to agree upon by historians. and too often subjected to game dev's discretions. and not even the same.
particularly at the break between Middle Ages and Renaissance/Early Modern.
1. in Civ5 Renaissance begins in 1400 AD
2. in Civ6 it begins at 1350 AD (When Anglo-French 'Hundread Years War' still rages on, actually at an early stage with gunpowder weaponry being tested in the field).
3. Naval Historian marked the breaks at 1450. (roughtly by the time Ottomans took Constantinople and its leader claimed the throne of Old Roman Empire). or later (When Martin Luther nailed his 95 Thesis to the front of his church building. criticizing Roman Catholic Church of indulgence sales to the core.) citing that
- Carrack is Medieval design
- Caravel is Early Modern design
I don't know who's right and who's wrong.
The other dispuited era breaks is 'When Industrial Era begun'?
(or even how 'Industrial Era' is defined. and wheter did 'Industrialization' and 'Industrial Revolution' are one and same. some wiseman doesn't agree that the two are. since some said any agrarian countries that got foreign industrial investments (non-agroindustry) doesn't considered 'industrialized' without the said country citizens or the governement successfully invented any technology, innovations, or inventions for industry. and this explains why such countries stagnated about a generation later after it has industrial investment booms early.)
1. In Civ5 Industrial Era begins late. about the same time as American Revolution War.
2. In Civ6 it begins VERY early (1725)
3. In Empire Total War it begins at 1700 AD
4. Wikipedia entry said the 'Industrial Era is 1760-1840' . it is 'Four Scores Long'
https://en.wikipedia.org/wiki/Industrial_Revolution
Each era has its own military history. and at the Early Modern this is when 'Everyone in the World Meets'. some societies that's still in 'Bronze Age' was either conquered by anyone with guns, OR did a big leap towards 'gunpowder era' without having to restructure its own society and politics. and with its own history it means in one era. a unit may be upgraded MORE THAN ONCE.
en.wikipedia.org
en.wikipedia.org
And this leaves 'Flintlock Military Era' in limbo. regardless how importance to the world history is (Modern Military is very much takes root from this era, and this includes military traditions of the two major branches (Army and Navy)). it never seems to get correct era settings in any grand 4X games.
1. There's no 'Fusiliers' in Civ5. this unit is represented either as 'Musketeers' or 'Riflemen' despite that this footslogger is a separate unit entirely (And Riflemen is quite a debatable in game designs whether did it deserves a separate unit entirely. since this unit was originally 'Fusiliers' with Minie Rifled Musketry, but it has double, triple, or even quadrouple firepower increases due to rifled standard weapons. and rendered oldschool Gentlemen Cavalry Tactics utterly useless. (It was once useful in the Napoleonic Wars, though some nations, like the United States of Amerca, looks forward in the future and thus not bothering invest their efforts training and maintaining (Enlightenment Era) Fancy Cuirassiers but instead developing gunny dirty ragtag 'Cowboy' cavalry)
2. Civ6 Very Late Patch. There's 'Line Infantry' dressed in the 1850s style uniform (or Late Napoleonics to be earliest, outer coat with no tails, shakoes that later looks like peak cap), this is 'Fusiliers' (one and same actually)
Also in the sea. there are indeed significant naval developments that worths its own 'era'. between 1450s (Caravels, Carracks, and Galleons) and 1860s (Steamships, Ironclads and steelhull vessels), there were naval developments that concurrents with ones on the land. note that warships of 1450s and 1700s are significantly different in shapes and performances. For Example. Felipe II's Gran Armada and English Royal Navy consisted of anything that can carry no bigger than 50 big guns. the biggest being Galleon. While Navy of 1760s have ships that's thrice the lenghts of Galleons.
This means. HMS Victory could easily shred big war galleons of a previous century, yet is not a match to Caio Duilio of 1870. and the same would do if US Navy deployed USS Constitution against Confederate CSS Virginia (itself a rebuilt frigate) in 1862. instead of newly developed USS Monitor.
Then again using era. placement of these units are indeed in limbo or requires its own 'era'.
Actually when should an Earlymodern-Industrial break tooks place? (or even there should be 'an era between' (which only valids for West Europe and Continental America, not anywhere else)
A. 1650 AD
B. 1700 AD
C. 1760 or 1776 AD
D. 1800 AD
The only thing that's clear is that. 'Bombards or Cannons comes BEFORE Musketeers'.
Hope Firaxis got it right next time