This seems like a persistent illusion to me. As access to the Internet and telecommunications reach has expanded, and language barriers or at least translation barriers decreased, any culture whatsoever, including whatever you might call "youth culture," is broader reaching and more uniform than ever. Hell, I'm not that old, last being a minor in 1997, but even then I barely knew what kids in the next school district were into. Radio was not national. I grew up in California and my wife in New Jersey and what was popular to us was often quite regional to the point I had never even heard of bands that were huge to her and vice versa. Television was more national but even that is only "national." Whatever got broadcast in the US was not broadcast in any other country that I'm aware of. Hollywood films were exported but not to the extent they are today. A few very popular, usually English-speaking bands became international hits, but again, that was exceedingly rare. The vast majority of what got popular in Germany or India I'd have never been aware existed.

Even Reddit was never really the front page of the Internet. That's a marketing slogan. I was pretty heavily online from at least 2003 to 2010 or so and never visited Reddit at all. I have no idea if any of my friends and other acquaintances did, but it was nothing they ever talked about to the point that I felt like I was missing out, and that includes friends I only knew from the Internet.

Go back before film and television existed and then what? You think there was any kind of meaningful cultural similarities between young people in Russia and young people in Argentina in 1890?

Where I think the disconnect you feel is very much real is that communities that cut across generations have largely disappeared. In the past, they may have been hyperlocalized, but they were real. 80 year-olds and 16 year-olds regularly interacted with each other, whether that be at church, service jobs, multigenerational housing, or simply that people visited each other in-person more often, if only because those visits were often only around the block. An old person in 1780 may not have had any clue what anyone was up to in another country or even another subnational district, but they probably felt like they knew their own grandchildren. I certainly spent quite a bit more time in the 80s hanging out with and talking to my aunts and uncles than I currently spend with any of my nephews and nieces.

The "pretty distinct cultural trends and identity" you're thinking of are what happened to get recorded by historians and/or make it into a popular Hollywood movie. It's very far from everything. As just a simply example I can think of from my own childhood, it became huge for a few years in the late 80s for kids to pull the magnets out of large speakers and use them to sift iron from sandboxes at playgrounds and schools. The idea was to trade in the metal to hobby shops for cash. Talk about it to any kid from my school district in 1989 and I guarantee they'd know what you were talking about. This wasn't obscure middle of nowhere. I lived 10 miles from downtown Los Angeles. Yet I have never seen this depicted in extant media, can find no evidence on the Internet it ever happened, and have never talked to anyone I didn't grow up with that would know what I'm talking about if I mentioned it.