So your telling me that a woman is only fertile for 20-25 years? That is completely incorrect women can bare children on average from 12-60 years old, and in some cases even younger and older.
Most women hit child bearing age on average of at 13 years old. But rarely is she married off right away... only in extremely primitive cultures, and the Israeli culture of the old testament doesn't strike me as particularly primitive..
So let's say she's married at 16 or 17. Children will come faster at the beginning of a relationship, but by the time your working the land, and raising 3 or 4 small children, things wane somewhat in the romance category.
And remember, the Israelites worked their land, and women spent on average of 2 hours a day just grinding the grain into a useable product. Let alone the cooking, cleaning, doing laundry etc.
Even with modern conveniences and her mothers help my daughter in law is exhausted at the end of the day with 3 small children @ the age of 24. Now take into consideration a lack of running water or any other convenience in ancient Israel, and there just wasn't a lot of energy at the end of the day..
So at one point romance begins to wane, and children aren't born as quickly, just naturally. ...
You rarely hear of any woman who has many more children than 12. My husband has a great great grandmother who did bear 18.. but that's a very rare thing. None of the women in my family could have that many (although the death rate in my family has always been high for the mothers) So I can't look back in the history of the women in my family and find more than 7, ever...
Some women can bear more than 12, but usually it's right around 10-12 when a woman's bodies gives up on her and she will go into an early menopause or start losing children before their time...
Even in Africa where birth control isn't practiced the average is 20, but not all are live births. At one point a woman's body is simply done.