[Saulot]

This is simple.

Make a mod. Key text in the mod should include:

#selectnation 32
#era 3
#end

Use era 3 for late age, era 1 for early. Activate the mod anytime you want to play the game that way.

Read the modding.pdf in your doc folder, or get the improved modding manual available on this forum, to understand modding basics.