Commentary for Genesis 17:24

בהמלו WHEN HE WAS CIRCUMCISED — a Niphal form (passive), like (Genesis 2:4) בהבראם “when they were created”.

ואברהם בן תשעים ותשע שנה, when he received the most recent prophetic vision he was 99 years old as already mentioned in verse 1. This had been at the end of 99 years after his birth, so that when he was circumcised he was still 99 years of age, and had not entered the 100th years. The Torah writes this in order to reinforce the report that he carried out the circumcision with dispatch, and did not wait until he had turned 100 years old.