(Bloomberg Opinion) — The generational divide is described and defined in many different ways, some more empirical than others. Many of these theories are speculative rather than confirmed, so in that spirit I would like to offer my nomination for what is truly the dividing force: screen size.

In a nutshell, younger people today are very comfortable with a small screen and older people are not. Both younger and older people can be found staring at their phones for texts or email or directions, but the big difference comes in cultural consumption.

According to one study, the median age of an American television viewer is about 56, whereas for mobile and computer video viewers the median age is 40. Forty percent of those viewers are between 13 and 34.

I am frequently stunned, but no longer surprised, by how many of my older friends and acquaintances do not appreciate the reach and power of YouTube. The 10 most popular television series attract between 12 and 20 million viewers per episode, but the top 10 YouTube channels all have at least 40 million subscribers.

And that doesn’t even count quirky channels such as Korean Englishman, which would never get greenlighted on TV (it’s bilingual and consists mostly of videos of British people reacting to Korean food) but has more than 3.4 million subscribers.

I’ve found I can travel around the world and ask (younger) people what they watch on YouTube and pretty much always get an answer. When I was in Lalibela, Ethiopia, last year, my guide was an expert on early Armenian church history, which he learned about by watching YouTube on his mobile device.

And it’s not just YouTube. According to Wikipedia, there are 40 Instagram accounts with at least 50 million followers. Instagram is largely a mobile-based service, like YouTube, and over two-thirds of its audience is age 34 or younger. These same young people popularized cord-cutting, and many use the smaller-screen internet to watch television events and programs.

I am obligated, as a 57-year-old, to point out that the viewing experience on a mobile device — or even a laptop — hardly comes close to watching in the glamour movie palaces of earlier generations. I have been watching new movies on a large screen since the 1970s, and in my youth I regularly visited repertory theatres that showed earlier classics. So I can say with confidence that Alfred Hitchcock’s “Rear Window” is one of the glories of American cinema — and no more than a pretty good movie when watched on a living-room television. If you have only seen “The Godfather” or “2001: A Space Odyssey” or “Barry Lyndon” on TV (or, worse yet, on an airplane or mobile phone), I would say you don’t know those movies at all.

Just as many older people don’t grasp the import of YouTube, most younger people have a weak sense of the power of cinema on a large screen. It’s not entirely their fault. It’s relatively easy to see older movies on a big screen in London or Paris, and maybe in New York City and Los Angeles (and Silver Spring, Maryland, home to the American Film Institute). In most other places in America, it’s much more difficult.

Sadly, the world is rapidly becoming a place where cinematic history, as it was created for larger screens, no longer exists. Netflix, for all its wonders and diverse contemporary selection, is notoriously bad about making older movies available for streaming, and at any rate the service does not provide a properly large screen for those films.

The situation with music is analogous, but instead of screen size separating the generations, it’s bandwidth. Audiophiles are on the whole an aging collection of listeners, whereas Spotify and YouTube are remarkably convenient, even though as used by most people they offer sound quality much worse than the compact discs of the 1990s or even many of the long-playing records of the 1960s.

Once again, technology is enabling great gains in convenience and diversity. What is being lost is a sense of magnificence.

It is possible we will look back on the present day as a special time when both patterns of cultural consumption could be enjoyed in tandem and enriched each other. But I suspect not. As today’s over-50 crowd slowly passes away, and our experiences fade from collective memory, I wonder if the world might be in for a bigger cultural shock than we currently realize.