Avraham Avinu was told to have a Bris at 99 years of age. Why didn't Hashem have him do a Bris earlier?
1)He was not an Arul until Hashem commanded so there was no Mitzvah to do it.
2) With no Command it is against Halacha to wound yourself
3) Since it is a Bris means convent it requires two Parties so until Hashem commanded him there was no second Party to make a Bris with
4)Rav Nissan Alpert suggests Avram did Kiruv he had to relate to others if they knew he was DIFFERENT then he would lose his ability to relate to all men so he waited once he had to have a son Hashem commanded him him since now his position as father took precedence.
Why was he (Avraham) circumcised only at the age of 99? To teach converts; that if a man comes to convert he should not say “I am an old man, so how can I convert?” He should rather learn from Avraham who circumcised himself at the age of 99!
Chizkuni writes at the end of Parshas Lech Lecha:
ומפני מה לא נמול משעה שהכיר את בוראו כדי שלא תנעול דלת בפני גרים שלא יאמרו מיד שבא אצלו ציערו ומפני מה מברית בין הבתרים כדי שיצא יצחק מטפה כשרה וקדושה
And why was he not circumcised from the moment he recognized his creator? So as to not lock the door in front of converts, so that they should not say "as soon as he came to Him, He hurt him". And why from the Covenant Between the Parts? So as to have Yitzchak come forth from a pure and holy drop.