Stellaris effect set_primitive_age_effect = command

This command changes the age of a pre-FTL civilization to <span class="parameter parameter-primary">Age ID</span>.
Command syntax
effect set_primitive_age_effect = <span class="parameter parameter-primary">Age ID</span>
How to open the console in Stellaris
Press the ~ / ` key to open the command console. This key is located at the top left of the keyboard, usually under the F1 or ESC key. Type the command and press the Enter key.