The chromosomes themselves aren’t even in error. The ovum is older than the average and divides in a funny way. Usually. It could also be the sperm that is too old (sorry, guys).
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.