How long were the childbearing years of the patriarchs before the flood?

In the second genealogy, Genesis 11, immediately after the flood, the average age at which a married man had his first child was about thirty-two; you can see it in the text, so-and-so begot so-and-so when he was thirty-some years old. Terah is commonly said to have had his first child at seventy, but that is false; his age was around thirty-two, like the others before him, Arphaxad at thirty-five, Shelah at thirty, Eber at thirty-four, Peleg at thirty, Reu at thirty-two, Serug at thirty, Nahor at twenty-nine. They all started their families young even though some lived to two or four hundred. The pre-flood patriarchs, Adam, Seth, Enosh, Cainan and the rest, started their families on average only about seventy-five to a hundred years later; add seventy-five to thirty and you get about a hundred and five to a hundred and thirty, which is why Adam had his first child at a hundred and thirty and Seth at a hundred and five. The highest is Methuselah at a hundred and eighty-seven and the lowest Mahalalel at sixty-five, only about thirty years above the post-flood figures; Noah is the exception, having his children late, but he is a special case because he had to build the ark. So it is not up to four hundred and fifty years; the genealogies only tell us when they started their families, because the names are given as calendar reference patriarchs, and we cannot know how many children they had or the full extent of their childbearing years. So four hundred and fifty or thirty-five is just a guess.