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?