Stage transitions are going to be very soft, the stages just exist to allow talking about specific parts of the game. Well at least when everyone has read the exact stage boundary definitions that exist on our wiki for all the stages (these have been basically set for almost a decade now with only very minor changes). For example for the society stage:
The Society Stage begins when your first Society Center (settlement) is built. Collective Residence enables the forming of Society Centers.
The stage ends once the player unlocks and builds the first industrial machine.