Firebrand - not sure of the science or relevance of your post.
Many scientists take the position that a woman has X number of eggs in their ovaries at sexual maturity, and that’s a finite number, whereas spermatogenesis is an ongoing process, and pending sexual activity, human sperm are often well less than a week old, every time.
I welcome your thoughts.
It was a guess, and qualified as being highly improbable. It is a fact, though, that Downs can be due to the father’s age.
Another possibility is that it is inherited, as a minuscule number of Downs cases are, and we just haven’t noticed it since it is very rare and we don’t test everyone as a matter of course. If so, it would most probably be recessive, since the parents of the two people were obviously fine.