My aunt wanted to "wait for the right guy" and "wait until she was financially ready".
She clearly doesn't have a low IQ, she graduated from Princeton with a degree in business. But throughout her 20s, no man was good enough for her and certainly not good enough to have children with. Finally when she was 35 she met a guy who was up to her standards. 8 years younger than her, Masters from Yale and made 6 figures.
Still they waited 3 years to get married and waited another 3 to have a kid.
They had 3 kids (my cousins). One when she was 41, one when she was 44 and one when she was 46. All male.
Now keep in mind, These are two upper class Ivy league grads homeowners in an affluent area. You would think they would end at least okay. NOPE!
The oldest (born when she was 41) - seems to be neurotypical but with a pretty low IQ. He was in special education for most of his schooling and they kept trying to figure out "what was wrong with him" as he seemed normal but very slow.
second born son (born when she was 44) - "high functioning" autist. Also slow, and low IQ.
Third born son (born when she was 46) - has down syndrome, don't need to say anything else.
Seems like the age of the parents (especially the mother) has more to do with how the children turn out than anything else. It might even be more important than the genetics of the parents and it's certainly far more important than the upbringing and wealth of the parents.