Trisomic diseases that result from non-disjuction of chromosomes during meiosis become more prevalent as a woman ages more. However, if all the eggs are formed in a woman early in life, why would these diseases become prevalent at a later age? Shouldn’t it be a random selection of which egg is released?
You are 100% correct in saying that a woman’s full complement of eggs is with her at birth.
What’s not so obvious is that the eggs aren’t fully developed. Prior to being fertilized they must complete 2 sets of chromosomal divisions (meiosis). Your average egg, just hanging out in your average ovary, is stuck in the middle of the first phase of meiosis. Upon ovulation, the first phase is completed. The second phase isn’t completed until just before fertilization (or right at the time of fertilization-I can’t remember which).
Anyhow, as you can probably guess, researchers infer an age-related defect in the ability to correctly segregate chromosomes in the maturing oocyte.
The woman is not always the source of the extra chromosome. In an estimated 25% of cases of trisomy-21, the extra chromosome comes from the man. I don’t know for sure, but I suspect older men are more likely to supply an extra chromosome than younger men.