You can't be unreasonable for feeling as you do...no one can help their feelings and it sounds as though yours are at least partially justified.
However I don't think your theory would apply universally.
I'm 38; my health is alright but there are things that are worse than average for my age. My children were born when I was 29 and 33.
My partner is in his fifties. We have not been together for very long and we would like to have another child.
What you have posted has tied into what I am thinking, as is he - he doesn't want to be difficult for us to care for when he is old.
I don't know if it would be right for us to have a child now. But we both want to and would obviously love it very very much.
One thing I'll say is that I knew a few people whose parents were older at school. Sometimes the father, sometimes both parents. I envied them.
My folks had us at 21 and 19, 23 and 21 and they were not emotionally healthy at all, and lots of things were wrong. My sister and I often felt older than them in certain ways. However, we got through and as they have matured, they have become far better parents to us than they were when we were small.
I envied my friends as their parents were all set; they had grown up properly, had lives that were good and sound and weren't phased by small children and their behaviour. They were proper grown ups.
I think it is really important to involve consideration of mental health and maturity in this process. These can often improve with age (though don't always) and this can balance the physical decline many of us experience.
I have known some very sorted children with older parents AND far older (next generation) siblings too. It seems the more older people there are around, the better balanced the small child being brought up among them.
Anyway I am sorry you feel so sad about it and thankyou for posting. I will have to think some more about our situation.