Educated people, who tend to be smarter, tend to have kids when they're older.
And kids with higher IQs are less likely to have mothers who are single parents. Again, causation or correlation? Does being raised by a single parent lower your IQ OR are mothers with higher IQs (thinking nature instead of nurture)more likely to wait until they are in a stable relationship to become pregnant. And on that thought, tying in with the first part on education, more likely to wait until they are finished with their education and established in a career before beginning a family.
In any event, DD was born when I was 26.. about 1.5 years after graduating from my second BSc and 1 year after getting married. DS was born when I was 32.. 1 year after I started my Masters. If he ends up being GT, I will squarely chalk that up to the fact that he started Masters level classes at the young age of 13 days old. :-)