There is a famous question asked: Since Avraham Avinu observed all the mitzvos of his own accord, (kedushin 82a, Yevamos 28b) Why did he not circumcise himself before Hashem commanded him to at the age of 99?
There are many answers given. I will list some of those which I remember (I do not recall the sources):
1- גדול המצווה ועושה ממי שאינו מצווה ועושה - Greater is one who is commanded and observes a mitzvah, than one who is not commanded and does it. Since Milah can only be done once, therefore Avraham waited until he was commanded.
2- Avraham was forbidden to circumsize himself because even a non-Jew is forbidden to do damage to his body. חובל בעצמו - As long as he didn't have the mitzvah it would have been an aveira.
3- A "bris" is a covenant. As long as Hashem did not tell him of the bris there was no covenant.
4- R' Chaim brisker said that a bris can only be done on an ערל. Until the mitzvah of milah, Avraham was not considered an ערל.
5- The מהרש"א says that Avraham only began doing mitzvos after his bris milah.
There are other reasons, and some known difficulties with some of these reasons, which I don't presently recall.
Question: Since Avraham knew what the Mitzvos would be, and what their details were, Maybe he also knew that (just as Yitzchok's mitzvah of milah was to do it at 8 days old, and Yishmael's mitzvah was to do it at 13 years old) he may have known that his own mitzvah of milah was only for him to do it at the age of 99 years old, and therefore no mitzvah of milah existed for him at all until the age of 99?