For sure age has nothing to do with the ability to read long texts, but generations have a lot of influence on it. It's not that all people of a generation read more than all people of another generation, it's that among people reading more, one generation will have read a lot more than another generation, and will have read a lot more long text without any graphic than another generation.
There's no complex reasoning behind it, just simple facts. No video games, no TV, no comics, no internet, no mobiles, watch movies is necessarily a rare organized event. This lets not much, sport, gardening, talking with people, DIY, cooking, a few more. None of those activities replace reading, paper diaries with long texts, books with even more text.
Argue some people were proud to not know read is a bit funny, I don't know from what country you are, but in mine even 60 years ago, that's plain wrong. But it's not the point, the point is there are people since young that are more inclined to cerebral activities, and more interested in realms/stories/dreams/more. There's perhaps more of them for younger generations, but for the older generations, they only had books and diaries so long texts. Believe it set up the same reading ability, ease, and enjoyment for those very different generations is nonsense, but for a tiny minority of mostly genius people.
There's the first schism, before TV and mass media, I skip older evolutions as books for elite versus books for the masses, and more. There's no match, even if it allowed more people interested in cerebral activities, it's still people reading a lot less because doing a lot of activities that are in competition with reading long texts.
Then came the Internet and mass internet. There's clearly a huge evolution, with a lot of people reading a lot more, but this phase didn't last long as the Internet becomes quickly very graphics and then video ready. Still, you have people reading a lot more, but nothing related to reading books, and one more activity in competition with reading really long texts without graphic supports, books. No proof, but despite reading a lot more, it's still the newer generation reading a lot less wide texts without any graphic support.
Then came youtube, mobiles, and social media. A part of it is involving reading more, but youtube lowered a lot of reading amount for many people replacing text reviews with youtube reviews and many more examples of long text reading replaced by something else (tutorial, learning, more). Moreover, those newer generations had to endure even more through social media pushing to short texts, even forums where posts more than 3 lines can generate complaints and aggressivity as if ignore is so difficult.
Even in work, there's been a huge evolution, and I see coming days where technical documentation will need to include graphics and many visual tricks to avoid that experts face walls of texts, and at a point, the video will be required too.
Now formations and information can't be texts and need include many graphics if not pure video, even if those graphics are totally irrelevant or stupidly superficial. Technical isn't yet fully involved but already started to register and manage the forced change. And I would say that a day or another, forums too will have to evolve to that way, and not just stick to whining when posts have more than two lines.