Will it be like in Tropico 5, where when you reach certain goal you click button and your screen would load maybe, and everything you have build will stay, but landscape, vegetation, animals, people would change, youd unlock new buildings for new age, and old age building would be gone or obsolete.
Or will it be continuous progress, without stop, land, people, animals, will just change itself, you start 3000 BC and suddenly its 2000 BC, and so on…?
I agree with you that some games are kinda weird with changes between ages. I remember some older games than Tropico to be exact. Age of Empire 1 and Empire Earth 1 are the ones I was thinking about.
Maybe a less mecanical changes would be nice. I have thought about this earlier and this is what I came to:
After you’ve made some research on how to build a house with Stone and prehistoric cement (or mortar) instead of wood because wood can burst into flames, you need to build a certain amount of stone house to be able to achieve one task on many to go to the following age. But don’t think you’re forced to build now stone houses. You should have the choice between wood house and stone house because stone can be hard to mine if you haven’t discovered better tools than wood ones. If for any reason a disaster happened, you will be glad to have the possibility to rebuild your village for your tribes with wood hut in less than a week than with stone.
Thanks to these, the changes between age is much more realistic and less magical. It also brings limitation because if you’re not in the next age, the ability to build a roof with brick instead of straw is not available even if you discover pottery.
Here is as I see the technology tree to represent these ideas:
I hope I have understood this correctly.
Anyone who is able to build stone houses will also be able to build a house made of wood. The choice is more likely to be more appropriate and which tools and materials are at his disposal.
Using research trees is such a thing. They are usually very static and clumsy. I think the advances in this age were more fluid in their own territory. Great progress has been brought by migrants as a “gift”. The focus should be on “person” and less on “research”.
The concept of age does not really exist in the game. They are there as way to group techs and content that come with expansions. But there is not a “You are in the bronze age” in game. You will not get all techs in a expansion at the same time so transitions occur in a natural when you have new techs available.
@UncasualGames that’s good to hear than the typical HUD thing ith “bronze age” is not in your todo task 
But I want to come back to something that @tschuschi said "Great progress has been brought by migrants as a ‘gift’ " it remembers me a very old game called Homeworld (out in 1999, it’s a Sci-fi RTS, I recommand it) where the Bentusi is an advanced race and they give you free technology to help you defeat your common ennemy. I don’t very like this kind of things because if in that typical them it could work but in AC, it would be kinda weird. I mean, I kinda like farming and take my time instead of getting a free technology from a random person who came visiting the tribe once per year. But that’s my opinion because I like long games instead of short game very intense like starcraft is.
In reality 99.99% of tech are always imported one way or another and even so it took thousands of years to propagate farming across the world. How it happens and at what ratio it happens are different matters.
I agree with you but at this time what happened accross the sea won’t come bother you so the tech difference between tribes won’t be so huge. I mean on a certains continent, people are quite all equals to a certain technology even if there is some exchange somtimes when people left their own to discover the world. Anyway, it is something you read somewhere ? Because if it’s the case I’m ready to change my position
Looking back we define the Bronze Age by particular technologies, though for the people of that era it was what it was. Thus, we live in one ‘Age’ so to speak and technologies come and go.
@UncasualGames has hit the nail on the head with the logic they have implemented for technology.