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.