I think there are two main factors: debt and the job market.
When, say, my mother was a child, you couldn’t just up and get credit out of the blue. Lenders were much more conservative than they are today and didn’t just throw money at people. A woman couldn’t get a credit card without her father or her husband to help her get one (which sucked, of course).
Today 18 year olds are bombarded with credit card offers, and they never let up. I’m 27, wrecked my credit when I was 19 and I still get 2-3 credit card offers in the mail PER WEEK. Our society encourages debt. We want it NOW, the old fashioned idea of saving for it 'til you can pay cash just won’t do.
And then there’s the job market. What job market? Yeah, exactly. My grandparents worked for Union Carbide. Coupled with my grandfather’s income that he earned from moonlighting as an electrician, they did well. Not rich, but they were able to buy a house and raise 2 kids. Granny did work full time for years, but when my mom was a teen she quit working. Sometimes they were a dual income family, sometimes not. They could DO that. They had that option.
Granddaddy had a 6th grade education (he was a genius, though). He had no trouble finding a job. There were factories to work at, businesses were booming and it was basically a damn good time to be settling down.
Today? Not many factories, and the ones that do exist may provide good jobs, but the competition is so fierce. Unless you have some unusual talent that can provide you a living OR you are fortunate enough to inherit wealth or a family business, your ass had better get to college.
So we’ve got a situation in which college is pretty much your only bet for even a crummy desk job, and kids are coming out of school saddled not only with massive credit card debt but also owing Uncle Sam 20k or so.
My generation has fewer siblings to compete with for resources. Many of us are only children, and our parents planned it that way. Mom and Dad have room at their house. Jobs are scarce, so why try to pay rent, utilities, student loans AND credit card bills when you can just move back in and live with the 'rents for free?