There is a correlation between age of having a first baby and intelligence, not a causal link. It isn't because the parents are older that they have children with higher average intelligence (and it's an average before anyone jumps on this - of course lots of children born to younger mothers are as clever or cleverer) but that women who have children later have are on average more intelligent and highly educated themselves, as are their partners. Their children would have been equally intelligent if born to those women younger, but those women tend to stay in education and then work longer, which delays them having children.
So being older doesn't MAKE your children brighter, but older mothers are statistically more likely to have brighter children, if that's the age of their first child - doesn't make any difference if it's your subsequent children.
Again, many many factors make up favourable or unfavourable circumstances to grow up in/parenting of both children and adult children. Age is one of them. There are many others which will have equal or bigger impacts. And again, personal preference for having children younger or older, or knowing people who are finding parenting hard who are younger/older are just personal experiences and don't mean EVERYONE should make their decisions based on those observed patterns, or that everyone will have those same experiences.
I wonder how many of those who have been quite adamant that beyond 40 is a bad idea or even selfish, know many who became parents in this age group, or is there something about just being comfortable with what we know and our peer group?
It is interesting hearing from those adults who themselves were children of older parents, both those who hated it and those who found it no issue. I guess for current adults it was far less common. Give it another 10 or 20 years and it will be far more common for adults to have had older parents. We will then hear more about what it was like to grow up with a parent in their 50s and if dealing with health or death related issues in younger adulthood seemed much more traumatic than dealing with those issues a decade later.
Most situations are not ideal. We weigh the pros and cons and the risks and the liklihood of the risks becoming reality and we choose. And in the end, that pull to parenthood for many (not all) is very strong and will keep people pro-creating whilst it's physically possible. Older parenthood, with all its pros and cons will become more the norm and society will learn to live with those pros and cons, in the same way that it did when industrialisation meant people could afford to marry younger and so early parenthood became more prevalent.