And yet the same processes that induce fertilility reduction are still operational at 40 regardless of how many children you’ve had before.
Unless you’re implying that having babies in teens and 20’s somehow makes the eggs you have left in your 40’s somehow healthier. Then that would be an environmental effect, not genetic. Ie, the environmental differences between the two groups of eggs (40 somethings who’ve had many kids and 40 somethings pregnant with their first) are somehow inducing genetic defects in the latter group?
I’m not saying it’s fertility based at all. There may be a nature, age of the egg or age of the sperm component there, but as with many things, some nurture is probably involved.
Older parents who have experienced difficulty bearing children may behave differently than parents who have never experienced any difficulty having kids. Older first time parents may approach dealing with children differently and there is less chance that older parents will have large families.
Smaller families with parents used to dealing with adults primarily, who are fearful about all the dangerous things in the world that can happen to their child, may consciously or subconsciously contribute to more instances of children (predisposed to autism) developing autism.