And it's unreasonable to have never seen this if you're older?? I don't know about you, but I still discover new things despite being twice the quoted age.
His point is something along the lines of, the older you are the more likely that you've heard of it, especially if you have a history of reading HN much.
"More likely" is a far cry from "there's a lot of 16-year-olds around here" (paraphrased). There's no reason to think that's the cause rather than the alternate theory that there are new people joining HN all the time (as you suggest), or simply that even people who have seen this thing before still upvote it.
Despite having a curious mind I still find things I didn't know about. Being 42 I experience this while feeling a combination of frustration and delight. I am working on the silly frustration bit :)
BTW I read the book last summer for the first time. Its brilliant and the underlying message even more.
"BTW I read the book last summer for the first time."
In the proud tradition of founders generally solving problems that mostly affect themselves rather than the rest of the population, I propose the startup idea of a hackers bookshelf startup. Not the first time its been tried, its a hard problem which means its a good startup topic.
I've been reading HN regularly for the last few years and never seen this. I wouldn't be surprised if I've read popular things you haven't too, unless you're on here every second of every day.
Having never seen this before is perfectly reasonable if you're new.