I don’t see any differences with regard to age when I euthanise animals. Despite what some owners believe, I don’t think the animals have any concept of life and death as such, so they only fear strangers, pain, etc., and some have more or less fearful temperments. I have euthanised many dogs who were happily wagging their tail and licking me, as well as many that had to be sedated before they could be euthanised because they were extremely fearful, and I try very hard to avoid causing any more fear or suffering than I have to. Age doesn’t seem to be an issue with them.
I do think age affects how we feel about our own death. Young children have to gradually learn the intellectual concept of death, either through explanation, fiction, or experience. But I don’t think any meaningful, visceral sense of oneself as truly mortal sets in for most people until well into adulthood. The young feel invincible and immortable, though they may know rationally they are not, unless they have experiences which make them believe in their own frailty and mortality, which for most of us if we’re lucky doesn’t happen until the body starts degenerating noticeably (say mid 30s?). Obviously this is not always true, depending on one’s personal brushes with death.
Now, in my early 40s, death seems more real to me than when I was younger. I’m not sure that makes me more afraid of it, but I truly know it’s on its way, and that informs me feelings about the relationships and experiences I value. I can’t say personally if death is more or less frightening at greater ages (Occam?). I would think it might be, due to being likely nearer, but also perhaps less so for the same reason.
Do I really fear death, or just dying and the consequent loss of the people and experiences I value? I think when I say I fear death, I really mean the losses that go with it, and any attendant suffering, not the experience of being dead, which I suspect is the absence of experience and, as Occam points out, there is nothing in nothing to fear. So I guess my fear is “rational,” as you describe it, though that seems something of a contradiction. But I think part of the intial idea that started the thread was does fear of death lead people to religion. I can see how it would, if religion promises that we will not truly lose those things we fear to lose with death. But I think I value my life and regret the inevitablility of losing it as much as any believer, yet knowing that death is inevitable, and most likely a true ending of what I am, doesn’t lead me to try and escape it through comforting fables. I started life very open-minded about such fables, and my skepticism about them grew along with my growing acceptance of my mortality. So, as I suppose we all agree, fear of death
may be a factor in driving religious belief, but it does not have to be a factor.