The speed of population growth is very dependant on how old parents are when their children are born (the generational age difference).
The generational age difference cannot be accurately expressed as a single number because it must take into consideration that
firstborn and lastborn children have a different generational age difference.
Another difficulty in calculating the generational age difference is that it is not the same for fathers and
for mothers. The 2000 US census indicates that today, on average, wives are about 2.5 years younger
than their husbands. This indicates that women would be typically 2.5 years younger than their husband when their first child is born.
Given the long ages of Genesis five, it seems likely that the age difference between fathers and mothers might be even greater.
Furthermore, Genesis chapter four indicates that at least some ancient people were polygamists.
This typically leads to one wife being considerably younger than her husband.
Because mothers are younger than the fathers, the mothers' age when their first child is born is the one that determines how fast the population grows.
Unfortunately, Genesis provides very little indication of the age of the mothers.
Genesis chapter five does provide the age of the father when the named son is born. These ages vary from 65 to 187. However, all this tells us is that
the mother is likely under age 65, and perhaps far under, when her first child is born.
Some people read Genesis five as a list of first-born children. That idea comes from a misunderstanding of the text.
The text repeats this language for each generation with the name and years changed:
"When Seth had lived 105 years, he fathered Enosh.
Seth lived after he fathered Enosh 807 years and had other sons and daughters."
Some people read this as:
"When Seth had lived 105 years, he fathered Enosh.
Seth lived after he fathered Enosh 807 years and after he fathered Enosh he had all his other sons and daughters."
That is not what the text is saying. God is not giving a list of firstborn children. This list is not the firstborn of Adam but rather
the ancestors of Noah. There is a very important reason why Noah must
provide the names of each of his fathers, but that is a different story. The first son on the list, Seth is not a first born, but very likely a last born.
Abraham, Isaac, Jacob, and Judah are not firstborn. It is a misreading of the text to think that these are all firstborn.
What the text does indicate is that the age of the father varied greatly when the named son was born. Men can sire children when they are older,
but typically women do not give birth when they are older.
Thus, what we can take from this text is that the women were younger than 65 when their first child was born and possibly much younger.
Furthermore, every father had the named son and other sons and other daughters. Thus each family had at least five children (3 boys and two girls).
This seems to indicate that the actual number of children was more than five and perhaps much more.