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)

וְיִשְׁמָעֵ֣אל בְּנ֔וֹ בֶּן־שְׁלֹ֥שׁ עֶשְׂרֵ֖ה שָׁנָ֑ה בְּהִ֨מֹּל֔וֹ אֵ֖ת בְּשַׂ֥ר עׇרְלָתֽוֹ׃

And Yishma᾽el his son was thirteen years old, when he was circumcised in the flesh of his foreskin.