The Rambam’s opinion clearly differs from Rashi who writes clearly that only Keturah’s children had to be circumcised, but not her grandchildren (or their descendants).
Addendum: After having done some further research, I have found that not all Arabs or Muslims undergo circumcision at age 13. Some undergo it anywhere from age 6 to age 13, while others undergo it when they are seven days old. Some don’t undergo it at all since circumcision is not mentioned as a command anywhere in the Koran.
Nevertheless, our reader’s question is still basically a valid one: Why don’t Arabs circumcise according to the timeframe given to Abraham whom they claim as their ancestor? According to the Rambam, whom we just cited, the question is even stronger.
(To be continued)