Why do you expect that extending the child-bearing age will eugenically improve IQ? In other words, why do you think that there will be more high-IQ old mothers than low-IQ old mothers?

Part of that could be expecting current trends to continue--that is, higher IQ women are more likely to attempt to have children later in life, and because fertility is lower at higher ages, they end up having fewer children overall. If people stop having children because they have enough, rather than because they are biologically unable to have more, this should have at least a counter-dysgenic effect, though not necessarily a eugenic one.

I expect children have a diminishing marginal utility. Enough is enough, when all your free time and money is used up by them, even less intelligent people tend to stop. I think.