"The Motif Awakening of history itself. Loss is transformation and Age Crisis are the fabric the world uses to digest its own history to build a wiser future for the next generations."
An Ages transition point that ends when certain crisis technologies, or checks are triggered, such as population, Eventss, etc., and they're always what separates Ages regardless of the time period each comprised. There are many but they're always thematically aligned to World Events and the climax of pushing one Civilization limit by introducing tension through their own actions.
Gateway To Genesis implements Age Crisis that are triggered based on societal development and present challenges that usually benefit Underdogs. This is what in the real world the French revolution did for democracies, you don't have to deal with revolutionaries as the rest of the Civilizations that are endlessly enduring fights to protect their monarchy from the rebel units if you have the correct government type. Age Crisis are the main fix to snowball that has been plaguing the 4X genre by merging it with aspects of resets with the Incremental Clicker Genre.
Age Crisis never announce themselves, initially they look like the Events of an Act of Fate which tend to be milder versions of Age Crisis, such as a revolt, a plague, an outbreak of sorts, wars, etc. However, Age Crisis are much more potent because they tend to reach their climax when the entire world is breaking down. There isn't a big announcement saying "Age Crisis" when it begins, rather it is Civilization that realizes they're in an Age Crisis when everything is failing, mirror how real-world crisis function by being labelled as such not at the start but at their climax.
The most common of these examples is The Inescapable Hunger which begins with the even of A Brown Auric Peach? which is only a low quality harvest. However, this event is the first domino towards the end as it signals how the soil is giving away due to the growing population and unsustainable monocropping practices.
An Age Crisis never announces itself, you only know you're in an Age Crisis when it's too late to stop the momentum of compounding catastrophe from consuming the world.