Sforno on Genesis 21:5/ספורנו על בראשית כ״א:ה׳

1ואברהם בן מאת שנה, although as an old man we could have expected Avraham to delegate the task of performing his son’s circumcision, he did so himself, ignoring the fact that he was an old man. [the commentary is based on the fact that we knew that Avraham was 100 years old at the time, so why repeat it?]