This seems like an idea that has good roots to it (what is meant by "family" and how people live together has changed over time, things taken as absolutes today are not necessarily of ancient origin) but has been turned to fit a popular narrative (the Baby Boomers were the first to experience X and the only ones for whom X was practical, and then presumptuously forced everyone following after them to conform to it or be considered failures).

In the early days of English settlement in North America, family was defined as everyone living in a house, subject to the will of its head: usually a man, but occasionally a widow who hadn't remarried. This "everyone" would include the wife, unmarried children, servants and slaves, apprentices, and any other relatives who couldn't maintain their own place. Patriarchs effectively had total power over their dependents (though also often full blame when they broke the law or otherwise misbehaved), and the display of proper respect toward them was of great interest to the state.

By the end of the eighteenth century, family relations were on a much more affectionate level. This is where historians often pinpoint the origins of the conceptualizing of "family" as the nuclear family, with both parents doting on their children, and servants and slaves, if a family had them, as outsiders. At the same time, the point of adolescents' education was to make them more independent from parental control: older teens were beginning to be classified as "productive" on censuses, rather than "dependent" like their younger siblings, and novels and conduct literature emphasized the need for parents to treat children in these categories differently and to expect different amounts of slavish obedience from them. (Part of the change in how servants were perceived had to do with the changing demographics of that class. In earlier generations, it was normal for children of the middle classes to go out into the homes of relative social equals in order to learn household skills, but by this time the practice was dying out, and servants were more likely to be born poor.)

Around this time, a certain version of the concept of independence was really gaining steam, which bears a strong resemblance to the modern American one. An independent person was male, not a part of an irrational hierarchy ("all men are created equal"), and self-reliant in politics and work; he had the right to do what he wanted with his money and property. Basically, he was free from obligations to the community, while actually dependent on his wife and servants to do the social and physical work that would keep him able to do what he liked, and often required governmental or community assistance in some way that could be construed as what he/his family were entitled to in order to continue being self-reliant. This contradiction/problem would continue to underpin the American idea of individualism that is brought up in the critique of housing.

Part of the reason families during this time tended to live together, even beyond ideology, is that they tended to be working together. In rural locations, the entire family would be involved with agricultural and domestic production/maintenance. In small towns, they might practice a trade, with the father as head artisan and other family members doing necessary related tasks. During the Industrial Revolution of the early nineteenth century, parents and young children might be engaged in home production, while adolescents worked in mills and factories in related tasks, until home production was essentially wiped out.

Where we really see older children starting to move out of the family home is at the turn of the next century. Cities were attracting not just immigrants from foreign countries, but young adults from suburbs and small towns. They would work in offices and shops, and find accommodations in boarding houses and apartments; they and working-class city natives filled dance halls and invented dating culture. Earning a wage, they were able to support a somewhat independent existence that in many ways looked like the typical 20-something experience today. Even those who still lived with parents and siblings in order to contribute to the family budget demanded more time to themselves and more power in exchange for their financial contribution.

It's important to note that the situation described in the last sentence continued to exist through the twentieth century (and today). Very frequently, when people discuss things that happened to the Baby Boomer generation, they erase all but the white middle and upper classes involved. College enrollment rates increased through the 1960s, both due to the population boom and an increase in the proportion of people who could afford it; this is when the modern financial aid can be said to have begun. Unless students were attending college in their hometowns, this necessitated a certain amount of independence. Perhaps even more importantly, ages at first marriage also were rising from their very low lows in the immediate postwar period, so anyone who chose to live at home until marriage would be spending an extra few years there. And of course there is the culture gap between conservative adults who'd lived through the Depression and a world war and their children who had had a relatively easier time of it and who had more liberal views on recreational drug use, which presented an increased stimulus for moving out. However, working-class and middle-class young people in the recession of the late 1970s and early 1980s (the youngest Boomers and oldest Gen Xers) found it just as hard as their counterparts today to get profitable jobs and move out on their own.