Just thought about this (as I do from time to time). Now, I’m not talking about a historian’s definition. I’m talking about what is the earliest to you that you think of as society that you can see and think of as something contemporaneous instead of historical. Or maybe just what qualifies as modern enough to be modern. But mostly, when is the past no longer a foreign country - when is it more recognizable to you?
I expect it is going to vary wildly by age and by location.
For me, the absolute earliest is the 1920s, but more likely the 1930s. And that’s in the cities. What first made me think of this was reading and early where Superboy appears. I read (a digital copy of) Action Comics #1 ages ago. It’s in the 1930s, and no prob. But seeing teenage Clark driving a horse-drawn wagon was just off-putting. Strange. Superheroes do not belong to that era. Of course, cars already existed by the time it was set (as a practical device and not just a rich guy’s plaything), and horses were used, especially in rural areas, for a good while longer. The urban/rural divide was just ginormous in terms of running water, electricity, and other amenities. I know that. I’ve read about it. But those stories (early Superboy and early Superman) belong to very different eras to me, and even though both were decades before I was born, one seems very different than the other.
So much of this is, for me, more about how I think of the eras, rather than what the really were, and I am aware of that.
I do think a large part of it is that I could watch old movies from the 1940s when I was a kid (though only occasionally did). I’ve listed to some radio shows from the 1930s and 1940s. I have read some old comic books. And, while there are massive changes, the world looks so much more recognizable to me than earlier. And by the 1940, many non-rich Americans had refrigerators (where they had electricity), which is a big deal. Cars looked more like cars (old cars, but still cars, not like the 1910s earlier 1920s ones). That’s when the world looks like my world, just in the past, instead of like a foreign country. I’ve never watched or listened to anything from the 1920s. Read some Agatha Christie, but that’s about it.
Another point might be post WWII. More suburbs, and a huge increase in consumerism. Tons of contemporary media, new geopolitical realities and norms for the United States (a superpower, Cold War, etc.) that would shape the world into my childhood.
After that, I just sort of fall into the thinking of each decade by stereotype, with the ones I am old enough to remember being “not that long ago” and the media from the others being “old.”
I could definitely see the end of the Cold War (or it’s ebbing) as a shifting point for those a bit older than me or my age, but from different places.
I can see 9/11 as a turning point for “modern” for younger people. Similar timeframe for the ubiquitousness of cellphones. But I just don’t know what, that is newer, that might be considered as a shift to modern times, at least for an American, I don’t know. It’s too old for me.