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.
