Having your first child in your 40’s is a new thing. You’re correct about women continuing to have children into the 40’s but most of them started in their teens and 20’s. Women who did not have children by the time they were thirty to forty were considered barren if married and old maids if single.
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?