I suspect that this is going to become a long term trend in the US, for several reasons.
To start with, there are inherently too many college educated people for the available jobs, that would allow them to maintain, or even achieve through hard work, a standard of living approaching that of their parents.
Add to this an immense level of debt to get this education, that effectively displaces 10 years or more of earnings that would otherwise be used to start a family and raise children.
Finally, the prosperity level of their parents, in many cases, is such that their parents can easily support them, in exchange for “retirement services”, such as being a chauffeur, cook, house cleaner, and other labor.
But this comes under “Amah rules”. In old China, respectable women whose families could not afford dowry, could become “Amah”, in which they would live with another family, and perform light work, like cooking, laundry, and being a nanny. They were not, however, servants as such, and would be treated as a beloved spinster aunt, a member of the family.
However, they could neither date nor marry, nor have children, as a condition of their residence. And this also applies to American children who live with their parents.
Already, Japan has an enormous number of such children living with their parents, caused by economic conditions much like those in the US.
Eventually, conditions will change in the economy, that will make it far more viable for young people to be able to earn a living and start a family on the natural timetable, but those who live with their parents are, and will remain, a lost generation.
Thanks in large part to the bloated and helpful federal government.
The U.S. already has its own version of “Amah rules.” It’s called Social Security and Medicare, and it’s ultimately going to drain the resources of every taxpayer in the nation.