Modcomp/mod for 3.19 that makes it possible to limit which eras a civ can start in.
Basically, if you pick a custom game, and start in a later era, only civs determined to be appropriate to that era will be in the game. Or, put another way, it allows a modder to set which eras a civ can appear in.
Now separated from other material and works better than before: when playing a custom game the list of available leaders and civs changes right away when you change the start era.
Era Civs 2016-10-05