collapse of the old society

I don't think that's it either. Young adults have been living with their parents for generations and in many parts of the world that is still the norm. You just go from dependent, to independent, to caretaker of your parents as they age. My own opinion on this is 1) social media. Before we only compared ourselves to our immediate peers. Now we are exposed to millions of people online. Especially when combined with the insidious algorithms made to keep you on these platforms. 2) The constant feeling of impending doom that makes us feel like we don't have a future. Makes everything we do feel pointless.