People overusing their subscriptions are compensated by people underusing them.
It's true that the train company does some probability math and figures out some balance point for the proper price for the ticket based on some estimated bell curve of usage, just like a diner selling a "bottomless" cup of coffee for $1.
But that doesn't make the right half of the curve "overusing" any more than it makes the left half of the curve "underusing". They are all merely using the thing that the supplier sold in accordance with the terms set by the supplier.
Is it at a certain age, or is that only for children born latest at $YOUR_BIRTHYEAR?
They are pointing out that what the previous commenter said was ridiculous, by saying the same thing with merely other variables swapped in which more obviously illustrates how ridiculous the original statement was.
There is nothing ridiculous about the fact that people-that-use-more-than-average (over-users: p>.5) are compensated by people-that-use-less-than-average (under-users: p<.5) to establish an average... it's called spread.