What effect does galaxy age have during game generation?
Solution 1:
See the wiki.
Summarized, old galaxies have more binary, white and red dwarf systems, while young ones have more white and yellow stars. Planetwise this translates to more terran and jungle in young galaxies (white and red stars have the highest chance of containing those) while older galaxies tend to have more asteriods, gas giants as well as barren and lava planets.
Note, that this wiki entry is for Endless Space 1, the numbers might be a bit different for Endless Space 2, but the general tendencies remain the same.