I don't think this is true of the under-20s in western countries. Technologically, yes. Socially? Culturally? Mental-health-wise? Prospects of doing better than their parents? Not from the kids I talk to.
I think that's fairly unique in the last couple of centuries outside of certain religious groups with occasional end-times/moral-panic phases.
No, it didn't. That model of decline or cycle describes essentially every cultural viewpoint--the view of an inevitably inclining state of humanity is quite rare, and I'm not aware of anyone advancing that before the rise of humanism. It predates not only the fall of the Roman Empire, but the rise of the Roman Republic before it, probably predating even the Greek and other civilizations that arose out of the Bronze Age collapse.
Medieval civilization did live amongst the ruins of the earlier Roman civilization, but their experience did not originate the idea that humanity lives after the end of a golden age.
“Dark ages” is an oversimplification, but it contains a quite large grain of truth.