How old was Avraham when he sired children with Keturah, and was that unusual?

Question

After reporting Sarah's death and burial, B'reishit 25:1 tells us that Avraham had several children by Keturah. Avraham was 137 when Sarah died. Earlier (17:17), Avraham had laughed at the idea of having children at the age of 100, yet neither the text nor Rashi makes any comment about these later children.

Is there an interpretation that has this happening earlier, during Sarah's lifetime? If so, details please?

If the plain reading is correct, then was it unusual for a man of 137+ to sire children? If it was unusual, does anybody comment on it?

Answer 1

As far as I know, there are three approaches to this:

1. This was a natural occurrence. This is the opinion of Ramban, as is explained in this answer. This approach is also taken by R' David Tzvi Hoffman, who adds that given Avraham's lifespan at 175, his age at the time of marrying Keturah (about 140) is comparable to the age of 56 for someone of a more contemporary average lifespan (70-year). The only miracle in Yitzchak's birth is the fact that Sarah was previously barren, not having had a child in 90 years. [See here (p.383) and here (p. 262).]
2. This was a miraculous occurrence. This seems to be the approach of Abarbanel, who is bewildered at what possessed Avraham to do something like this. He discusses six reasons why God wanted Avraham to have more children at this late point in his life. (See here.)
3. This happened way earlier in Avraham's life, certainly long before Sarah's death. This is the opinion of Shadal. (See here.)

Answer 2

Ralbag addresses this in his commentary to Genesis 17:17 which you mentioned. There we find that Abraham laughed at the fact that he would have a child when he would be 100 years old and Sarah would be 90 years old. Ralbag there asserts that Abraham was only making a big deal out of the fact that Sarah would be 90 years old, but not that he would be 100 years old, and then tells us why he gives this explanation:

ואולם בארנו זה הפסוק באופן הזה לפי שכבר מצאנו באברהם שהיה בן קל"ז שנה כשמתה שרה ולקח אשה אחר זה והוליד ממנה ואין ספק שאילו לא היה ראוי אברהם להוליד לא היה נושא אשה כי חלילה לו שישאנה להנאת המשגל ואיננו זר גם בזמננו זה שיולד לבן מאה שנה וכל שכן שלא היה זה זר בחק אברהם שהיה ארוך החיים ממנו וכבר הבינו זה בזה האופן בב"ר

However, we have explained this verse in this fashion because we already find that Abraham was 137 years old when Sarah died, and he took a wife after this and had children through her. And there is no doubt that if Abraham had not been able to have children he would not have married, for heaven forfend that he would marry for sexual pleasure. And even in our times it is not outlandish for someone to father children at the age of 100; and certainly not with Abraham's constitution, as he was more long-lived than us. And this has already been understood in this fashion in Genesis Rabbah.

In other words, from the fact that Abraham was willing to marry at the age of 137, Ralbag derives that Abraham must have thought it quite possible for him to have kids at that age.

Thus, to answer your question, according to Ralbag's explanation Abraham's children from Keturah did indeed come at the age of 137, this was not tremendously out of the ordinary, and Abraham had not in fact laughed at his ability to have children at the age of 100.