Genesis 17:25

בהמולו את בשר ערלתו WHEN HE HAD THE FLESH OF HIS FORESKIN CIRCUMCISED — With Avraham, [the word], et is not stated [as it is here with Yishmael], because the only thing he was missing was the cutting of the flesh, as [his foreskin] was already flattened through sexual relations. But Yishmael - who was only a child - was required to cut his foreskin and open [and push back] the circumcision. Hence about him, et is stated. (Genesis Rabbah 47:8)

And Ishmael his son was thirteen years old when he circumcised the foreskin of his flesh.

וישמעאל בנו בן שלש עשרה שנה, seeing the Torah had mentioned Avraham's age at the time of his circumcision, Ishmael's age was also mentioned as a mark of honour for him. After all, we know how old Ishmael was at the time, the Torah having told us that Avraham his father was 86 years old when he was born (Genesis 15,16). Perhaps the Torah underlines the fact that Avraham circumcised his son Ishmael, as if he had not been circumcised by that age, he himself is obligated to either circumcise himself or to arrange for him circumcision. Ishmael was just young enough for this obligation still to be that of this father.

בהמלו את בשר ערלתו, seeing that the Torah added the word את here, a word which had not appeared when Avraham's circumcision is reported, the meaning of this verse is clearer than the previous verse. The word את, here, means the same as if the Torah had written מן, "from," i.e. "when his foreskin was removed from his male member." The word את is used as meaning "from" also in Genesis 44,4 הם יצאו את העיר, "when they had left the city, etc." Also in Kings I 15,23 the word את means "from," I.e. that the line חלה את רגליו, means that Asa's "ailment started from his feet." There is an allegorical explanation for our verse, mentioned by Rashi, according to which Avraham, whose flesh had become elastic, soft, due to his having engaged in sexual intercourse during many years of his life, did not need to have the membrane we referred to as פריעה removed, whereas his son who had not yet experienced the use of his male organ in a sexual function did need this membrane to be removed. According to this distinction, the word את in our verse would refer to something additional, as is customary with the word את. In this case the additional element was the removal of Ishmael's פריעה at the same time as the removal of his foreskin. The author of this explanation disagrees with the previously mentioned view of the sages who believe that at that time the command to perform פריעה had not been issued at all. I believe it is far more likely that the instruction to perform פריעה was given to Avraham at this time. Seeing that our sages state that anyone who has performed circumcision but has omitted to perform פריעה is viewed as if he had not performed any part of the commandment, (Shabbat 137) it is hard to accept that the first person performing this commandment should not have performed it in all its details, i.e. perfectly.