C. F. Keil and Franz Delitzsch on Genesis 17:22-27

Gen 17:22-27

When God had finished His address and ascended again, Abraham immediately fulfilled the covenant duty enjoined upon him, by circumcision himself on that very day, along with all the male members of his house. Because Ishmael was 13 years old when he was circumcised, the Arabs even now defer circumcision to a much later period than the Jews, generally till between the ages of 5 and 13, and frequently even till the 13th year.