There are a few things going on in the post you shared, so I'm going to take them one at a time. Starting from the top: No - the idea of moving out at 18 is not an artifact of World War II. Or, to put it another way, events or conditions that happened between 1939 and 1945 did not cause young people to suddenly leave home at 18. The idea that a young person would leave their parents' home had been established long before the war.

In Western culture, including America, it was normal for kids to live with their parents into adulthood and until marriage, sometimes longer.

"Normal" is a complicated, usually misleading, word to describe patterns among young people in history. That is, what's true for one group of young adult Americans isn't necessarily true for another group. The generational patterns among young adult Black Americans whose ancestors were brought to America through chattel slavery look different than young white men who have access to generational wealth which are different than Indigenous young adults. It took massive resistance from young disabled people to expand the social safety net to include them and the supports they need(ed) to live independently. Etc.

In America, that changed (for men) in the 40's and 50's when it became extremely easy for an 18 year old to get a job that paid more than enough to live comfortably on or even to afford college which would practically guarantee an even better job.

This statement does not accurately reflect patterns across history. In the early 1800s, thousands of unmarried women between the ages of 16ish and 24ish left their homes on the east coast to travel South and West to work as schoolteachers. Their housing situation ranged from living with a local family, in a room attached to a schoolhouse, to communal living. These young women didn't have to fight against social norms to move away from their parents; instead, there was a deliberate campaign led by advocates for public schools to shift social norms such that it became acceptable for young unmarried women - mostly white, but not always - to leave her home and live independently (while "fulfilling her natural obligation to the next generation" - more on that here.) Similar movement could be seen among young men (and women, mostly white, but not always) before the Civil War who followed the sentiment of, "go west, young man." This isn't to say all white young adults living in the Northeast left home to head West or South, only that it wasn't uncommon and a young person interested in doing such a thing may have faced some pressure from their parents to not go but the conditions were such that it wouldn't necessarily be seen as uncommon or as aberrant behavior.

I'm going to skip the next bit as it's modern politics.

In fact, a great deal of American culture is still based on the memory of the '40s and '50s as baseline normalcy despite them being a total fluke at the time.

I'm not sure what the author of this means by "fluke" but leaving the family farm became fairly routine and common beginning in the 1850s as the American economy transformed from an agricultural one to manufacturing. One study even described the consequence of this economic shift as "shattering" the system of intergenerational family farms that had endured for generations.^1 Specifics related to which child left home or why they left the family farm were idiosyncratic and more education didn't always mean a child would move away from their parents. A 1930 profile of American teachers by Frances Donovan identified a variety of archetypes, including unmarried teachers who received their Normal College diploma (a de facto teacher certification) and spent her days teaching and her morning and evenings caring for her parents. These women were of note to Donovan precisely because they stayed at home.

There is one more trend worth exploring that emerged before World War II and that's the idea that in many households, parents felt they simply didn't need their children's help. (Our friends over at /r/AskSocialScience are better positioned to discuss the why behind this than we are - or if it's a uniquely American trait.) Ruggles' study explores this theory in more depth and it pops up in a few other places. How Old Are You: Age Consciousness in American Culture by Chudacoff is one of my favorites on the topic of age and he explores the theme in terms of the construct of being elderly. In effect, he offers, World War I vets, couples who built their homes, adults who were otherwise living full, independent lives - whose children were adults - didn't appreciate the idea of their children, or their neighbors, thinking they needed help. (Although it wouldn't emerge until well after World War II, the sentiment "empty nester" can also mean the birds, as it were, were kicked out of the nest.)

So, finally, let's talk about why 18. We got lots of questions about the question of "why 18" and to summarize, it's because most young people start school at 5 or 6, are in school for 13 years, putting them at 17 or 18 when they're done. As America expanded the social safety net, established the draft, set legal boundaries around childhood and adult over the course of the 20th century, 18 became a bright line to mark the end of childhood (it's not necessarily a physical line as the changes associated with adolescence continue well into a person's 20s.)

So, to reiterate, no, not really. While more young white men did go to college as a result of the GI Bill and were able to obtain mortgages through special programs set up for vets, the norm of young white adults leaving their parents' home once they reached adulthood was established long before 1939.

---

1.Ruggles, S. (2007), The Decline of Intergenerational Coresidence in the United States, 1850 to 2000 link The entire article is worth reading as the author explores a variety of different theories related to why white and Black Americans leave home. (A tip of the hat to /u/jbdyer for putting it on my radar)