How is that "unnatural?" My mom had me after she turned 30, and my sister after 34.
By ‘unnatural’ I mean not the norm if rules were changed back to what they were for most of history.
It would be natural for women to already have several kids before 18 and a whole flock by early 20’s.
In the middle ages, this wasn’t all that uncommon. Menstruation was considered the coming of age for women. A lot of marriage happened in the teens even through the 1800s.
Today the average age in the US for women to have their first child is 28. That’s actually a LOT older than it used to be. In 1970 the average age was 21. In some countries in Europe it’s crossing age 30.
There are various pros and cons to this. But for most of human history, women were having children much younger than they are today.