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.