And Ishmael his son was thirteen years old, when he was circumcised in the flesh of his foreskin.
Read Chapter 17
Ambrose of Milan
Even the fact that Ishmael was circumcised in his thirteenth year is for an obvious reason, because one who is beginning to be sexually active should trim the ardor of his passion, so as to abstain from illicit unions and limit himself to a legitimate union only. .