One of the features of "extended adolescence" is that college feels less like a discrete life phase and more like a template for everything. An example: roommates. Everybody has roommates in college, but the idea of having roommates forever is a relatively recent one. The number of 25 to 34 year olds living with roommates increased by 39 percent from 2005 to 2015. The reason is pretty obvious: it's cheaper.
A lot of the commentary seems to suggest this is a temporary phenomenon: that at some point in the future, a better economy will lift millennials' miserable incomes, and they'll be able to afford to form single-family households like their parents.
More likely, however, is that this doesn't happen. When you look at the sectors where millennials are working (leisure, hospitality, retail, wholesale), their wages aren't going up anytime soon. But their rent will: as urban cores continue to reurbanize/gentrify, the cost of living is going to take a bigger and bigger bite of millennials' stagnant incomes. This means, inevitably, more roommates.
It might take awhile, but I'm pretty convinced that these economics will eventually make the single-family household obsolete. Living with roommates is just the beginning: there are going to be a lot more experiments in collective living over the next several decades. You already have "co-living" spaces popping up in New York and Syracuse: so-called "dorms for grownups," where people rent small apartments connected to a common area where they can watch movies, play games, cook group dinners, etc.
It actually doesn't sound so bad. The rise (or, more accurately, return) of collective living might be for all the wrong reasons, but it could have pretty okay consequences. As any punk rocker can tell you, the suburbs are lonely. The single-family household probably never made sense at a human or economic level, so I'm not sure we should mourn its demise. Still, homeownership is traditionally how people become middle-class in America. They get an asset that's supposed to appreciate indefinitely, and the federal government makes it possible with massive tax subsidies and the world's biggest socialized mortgage market. Excluding a generation of Americans from homeownership means a lot more than making them live with roommates forever--it also means barring them from the American government's biggest free-money machine, and the main route to middle-class wealth accumulation. It means their tax dollars go into helping their parents pay their mortgage, without ever being able to afford one of their own.