It's largely affluence. Individual rooms for each kids is relatively recent in American culture, and not always the rule. Particularly where people have limited space or limited means.

Even into the 80s and 90s, individual rooms was sort of aspirational.

It largely runs in tune with the rise of suburbs, and post war Great White flight. Goes a bit hand in hand with the reduction in the number of kids most people have. And particularly in cities where space is at a premium, it's still fairly common for kids to share bedrooms. Particularly when younger.

It's very tied to that "American Dream" idea of a large stand alone, suburban home with a white picket fence and defined yard.

My mother is one of 10 kids, she never had her own room as a child. My father is one of 3, and shared a room with his sister, and then his much younger brother (8 or 10 year gap). Until he was in his late teens and more or less finished himself a bedroom in the basement. His family had moved out of the Bronx, to a succession of ranch and town house style suburban homes when he was young. But they never owned more than a 3 bedroom. And while in the Bronx, there was only 1 bedroom for any and all kids.

I've got 3 siblings, and we all shared bedrooms till my family was affluent enough to put work into the house adding space and changing the layout to add bedrooms. With my parents first house, 3 kids were sharing 1 bedroom, and my youngest sibling was in parents room. That second house initially it was 2 kids per bedroom.

This was "big" for the family, as kids having their own rooms was for rich kids at that time. And in that area.

The expectation here that kids will always have their own space, their own rooms. Regardless of circumstance, more or less get fixed with the expansion of tract housing and the rise of McMansions. Basically from the 70s on suburban development houses get larger, and larger.

And the trend with home ownership shifts towards purchasing the largest home you can afford, values of homes linked to variety of rooms and number of rooms. Rather than more limited looks at what kind of space you actually need.

That coincides with shrinking family sizes, so the average family simply needs fewer rooms.

So by the 90s, it becomes fairly common in a lot of areas for home owners to just have an excess of rooms in their house. If you have a 5 bedroom house, and 3 kids. Well everyone can have their own room, and you still have an extra.

This largely doesn't take place in a lot of other areas of the world. Even affluent places like Europe. Simply because house sizes just didn't bloat like that, almost no one else went as hard on sprawl and suburbs as the US did. The culprit on that was largely cars.

As goes privacy. I'm less up on that. But from what I recall it tracks along a really similar time line. And for similar reasons. Expectations of personal privacy kinda develop a bit earlier, but largely as space that allowed for it in the first place became more accessible.

Applying that to kids, from what I understand comes largely with the Baby Boom. And the rise of pre-teens and teens as a cultural driver. So right around the same time all that suburban development and social mobility are going down. And that seems to be where keeping boys and girls separated enters the equation.

Prior to WWII it was apparently common for children to even share beds, and even multi bedroom houses to only have a single bathroom. So personal privacy was fairly restricted for pretty much everyone.

It is still fairly common for kids to share rooms, at least when young. Most of my friends kids and most of my nieces and nephews. Either share rooms, or were sharing rooms until recently.