I just attended the Great Bible Adventure seminar by Jeff Cavins this weekend and he mentioned that.
Under Period 2: The Patriarchs, when reading Genesis 17 Jeff explains that God renews his covenant with Abraham, but since he bore Ishmael thru bedding Hagar, the Egyptian - God expanded the practice of circumcision to all of Abraham’s people. This would have been in line with Ishmael turning 13 when he would have been circumcised had he been raised in Egypt…and then all the adults, of course, had to experience this procedure, while the children got the blessing of having it done by the 8th day after birth.
Anyway, that’s according to the notes I took at the time.
If anyone gets a chance to attend one of these seminars - it is WAY worth the cost!! I so want to bring the DVDs to my parish.