I'm comfortable with admitting that nobody knows where that point is (even if they think they do because of an old book they read). And I'm uncomfortable with imposing my personal feelings about where that point is onto other people.

We face this problem in all kinds of arenas, right? Age of consent laws for example. Different countries and states can't even agree on what the age of consent should be. And the age of consent for different things (drinking, sex, elective medical procedures) varies.

You and I might disagree on what point a child becomes an adult. So legally speaking, we pick what we hope is a sensible, but largely arbitrary line in the sand, and stick to it. Not because it's always 100% perfect, not because there's some clear, magical change at the age of 16 or 18 or 21, but because, given our dumb human brains and our inability to treat everything on a case-by-case basis, this is the best we can do.

And yes, arguments about the kind of life the child will have are relevant. First, I think that euthanasia should be legal. The goal of preserving life above all other considerations, even the needs of a person facing a lifetime of suffering, is morally indefensible as far as I’m concerned. Life is precious. I’m sure we agree on that. But a life of a few days filled with suffering? Personally, I don’t think so.

But secondly, we allow parents to make health decisions for their children. And when the doctor is telling them their child is going to die the moment they're born, or suffer for a few days and then die, it's insane to me that anybody would argue there shouldn't be an option to spare the child, and the parents, that suffering.