I can’t answer for all women; I might even not be able to answer for myself, as I’m in my late 20s now, and am only hypothesizing as to how I might be thinking in 15+ years.
While fertility certainly decreases in one’s 40s, statistically, chances of both miscarriage and birth defects increase. If I, historically, don’t handle miscarriage well on a mental/emotional level, I could morally make the decision that I am not willing to increase my risk of miscarriage, given what it does to me and my family. I could also morally decide that I have a different perspective than I did in my 20s, have more local family support, what-have-you, and so am willing to accept the increased risk of losing a baby and the associated grieving process.
Similarly, the various difficulties that parents of special needs kids (and, later, adults) face are not insignificant, though of course, like how one handles miscarrying, that’s also going to vary from couple to couple and circumstance to circumstance.
It’s unfortunate but true that with an increase in parental age comes an uptick in the likelihood of certain types of disabilities in children conceived during that timeframe, while also placing the associated realistic burdens, both resource-based and physical, on an older and physically-frailer parent: a parent who, at 45, has a child who requires certain types of care, is going to be lifting a 100+ pound adult as a 65-year-old, or perhaps dealing with early-onset Alzheimer’s in a child with Down Syndrome while dealing with their own aging issues.
This does not mean that a couple
must morally refrain from conception during this time, far from it! Again, that’s a decision that they and only they can make, but I’m sure that the above points are taken into consideration for a lot of couples in deciding whether or not they should try to conceive, try to avoid, or just accept what (and who

) may come. Lastly, as ever, God has the final say: one can always conceive a baby with Down Syndrome in one’s twenties, and no NFP method, total abstinence aside, has a 100% effectiveness rate. However, as humans without the gift of knowing the future, all we can do is educate ourselves as best we reasonably can about a decision, and ask God to help us make the right one given both the information at hand and our particular circumstances.