I think young sector has more hot planets and stars and asteroids belts, old has more nebulas, dim stars and gas giants, average has star systems with some hot planets and asteroid belts, some gas giants and nebulas, while mixed sector has all types of solar systems mixed, so there could be yellow dwarf with lots of asteroids right next to a red giant with only some remaining planets and nebulas everywhere.
The order of settings is bad, come to think about it. Player should choose in order: sector, difficulty, starting options. So, sector generation setting, then if they want ironman, pop-ups, normal or easy mode, then (depending on chosen difficulty) starting ships. This would allow to, for example, first time players to start with wolf and lasher (instead of a KITE).