Midrash Tanchuma (Bereshis 1:11) says: Rabbi Shimon son of Yehotzadak said, we learned that at the time that The Holy One Blessed Be He said to Adam "Cursed is the ground for your sake, in toil you shall eat all the days of your life..", Adam said, "Master of the world, until when?", (Hashem said) "until a man is born circumcised." Since Noach was born circumcised, immediately Lamech knew and said "Certainly this one will give us rest etc." Seth was also born circumcised (Midrash Tanchuma 9:7, other places). Since Seth wasn’t born yet at the time of the curse, shouldn’t the curse have been lifted when he was born circumcised?
The rule is, that one doesn’t ask questions regarding aggadita (Rambam, Moreh N’vuchim end of intro.; Nodeh B’Yehuda, Y.D. 161), simply because it’s not meant to be taken literally but rather usually meant to teach us a moral, or philosophical idea. Whenever you hear a rabbi asking such questions on a midrash or midrashim, it's because he also has a lesson or moral which he wants to give over through this age-old medium. Look in the commentaries on both midrashim and you'll see their moral.