Remember that not everyone is a direct line of first children. The average age of the mother of the birth of A child (contrary to her first child) seems to be around age 30 over most of history according to google.

Which does make sense if you consider that women would start getting children in their late teens (puberty used to start later) and end getting children in their 40s.

That's why for an average generation 30 years is usually taken.