Certainly things have been changed a lot since I worked on this file.
Ok, so I think I understand your question... let me hazard an answer.
I established the beginning of the game at 12000BC but after later research I think it should be about 300,000BC for the emergence of the Homo Sapien. We can debate that still a bit I'm sure.
Sumeria, the beginning of known civilization, arose in 6000BC so I considered 6000BC to be the start of the Ancient Era - I should review what techs we have set to progress a player to the ancient era before hazarding a guess which one should be the zero point concept there. I believe my wife and I usually find ourselves entering the ancient era with Mysticism, which does make some sense if we consider that the ability to spread the concept of a 'religion' at all was perhaps what made it possible to organize a society - that's what they were teaching in Soc 101 anyhow. (Of course, that religion was what we would call Mesopotamianism and mysticism and Animism would've come long long before that and Hinduism would've been hand in hand with the Caste System... but I haven't made those points loudly because we haven't paved the way for that conversation much.)
Anyhoo... then we have the Ancient era progressing to Classical and that'd be primarily earmarked by the rise and fall of Babylon, the emergence of Judaism and Monotheism, and Greek philosophy. Soooo... something in there. Not sure which tech to mark for all that but I think we're getting into some more rational (and perhaps because its better taught by our modern education systems and leaves less to theoretical conjecture) tech progressions at this point on our tech tree. Again, we should pull up the techs that are set to lead us into that era and figure out which is most central and humanly datable with some research. But I figure it'd be about year 2000BC.
Then the next category would be crossed at 0 AD and we jump right into the middle ages. And ok, sure Theology may be as others have said, widely recognized as being in practice around 25 AD or so or maybe even after, and the establishment of the Christian religion surely does come AFTER Christ was born and made his big impact in the world... BUT we can also just simplify and look at AD as After Death and BC as Before Christ so in that perspective, Theology makes a good zero point.
The middle ages was a very long and drawn out period where technology was vastly hindered by political systems that wanted to maintain control and had to strategize against the people's hunger for knowledge to keep them in the dark enough to keep them under the thumb of their dogmas. That said, we still took vast steps in some technologies, particularly that of war, as conflict was rife and constant as the common man served powerful masters who played complex games of court intrigue and dominance.
Then we enter the Rennaissance. Why? Because we began to throw off the shackles of the church dogmas in little ways here and there. This opened us up to finally spreading our wings as people in all areas of human development. As for WHEN this began to take place, it was probably most encapsulated in Martin Luther's break from the Catholic Church and that was about 1500 (1520 to be precise but in game measurements 1500 would fit best). What tech would that be? After some thought, Copernicus's life zeroed out at about 1500 as well, which would indicate that Astronomy could be considered the basic turning point that defined the entry into the Rennaissance.
The end of the Rennaissance saw the birth of the real Age of Exploration and Colonization, around 1700. I'd think Flintlock defined that - Musketmen...
Then around 1800 we see the birth of Representative Democracy (US). But that would take place in the same game era as above really so we can consider the assembly line and the introduction into the industrial era in about 1850 that really defines the next shift or so.
I'm not sure what I established from there in my considerations on game time... maybe that's a good place to start from for now.
I guess in summary, and to go look at Praetyre's concept above, I suggest something along the lines of:
But they're coming and yes, that's a big reason to do this!
Are you setting your research rates differently than what C2C has? Just wondering how you're "enforcing" the eras.