Suppose old person and child (perhaps better: young adult) would both gain 2 years, so we equalize payoff. What then? Why not be prioritarian at the margin of aggregate indifference?
* I've heard old people saying they wish they could become young again, but I haven't heard any young people saying they can't wait to become old.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.