Genesis 17:25

Ishmael … thirteen years old The mention of Ishmael and of his age, is of interest; for it implies (1) the fact that the Ishmaelite people practised circumcision; (2) the possible reminiscence of a variant custom by which it was performed at the age of thirteen years, instead of eight days; as in Israel, cf. Genesis 17:12. The modern Arabian use is said to be much later in life than that of the Jews, and in some cases corresponds with the age of Ishmael. A boy at 13 was regarded as on the threshold of manhood. Origen (Euseb. Praep. Evang. vi. 11) and Ambrose (de Abrah. ii. 348) mention fourteen as the age for the practice of the rite among the Egyptians.