The gemara 13b says that milah and korban pesach are the only two positive mitzvos for which there is kareis, but since they are violated by being passive - שב ואל תעשה, there isn't any korban for an accidental violation. The gemara explains that the concept of shogeig being chayev a korban chat'as applies only to violations similar to avoda zara which are violated actively, but doesn't apply to aveiros that are violated passively (even if done intentionally there is kareis). Tosafos 14 asks why do we need to exclude milah from korban due to passive nature of it's violation, there is a more fundamental difference between milah and avoda zara. When one violates avoda zara, at the moment of violation they are obligated a korban (b'shogeig), but by milah there is never a "moment of violation" since at any point he can do mila until he dies? Tosafos seems to be asking that since we can never considered him to have violated milah, because he always has the possibility of doing it, their should be no possibility of bringing a korban? Tosafos answers that the moment a person becomes bar mitzvah and didn't have milah, he is obligated in the mitzvah and immediately chayev kareis for not doing it. True, by doing it later in life he will remove the status of kareis, but until then he is in fact chayev chareis.
The Maharatz Chiyus asks on Tosafos, even though he is in immediate violation, how can we even suggest the possibility of bringing a korban for this violation? If he would bring a korban for the violation and then later decide to do milah, he would retroactively remove the violation so that the korban will be deemed chulin - how can we ever allow a person to bring a korban when it is not clear that he will be obligated in that korban?
The Rambam and Ra'avad (Milah 1:2) disagree about the kareis by milah. Their argument seems to be the question and answer of Tosafos. The Rambam holds like Tosafos assumed in their question that the kareis (and violation) of milah is at the point where a person dies without getting milah, but until that point he isn't chayev kareis since he still has the ability to do milah. The Ra'avad holds like Tosafos' answer that the moment he can do milah and doesn't, he is immediately chayev kareis, but has the potential to remove the kareis by doing milah later. The language that the Ra'avad uses to express his position is - וכל יום עומד באיסור כרת, similar to Tosafos' language.
We must understand the opinion of the Ra'avad/Tosafos. When a person does passes the age of bar mitzvah without mila and then gets milah at the age of 50 - do we consider that to retroactively remove the issur kareis, as if it were never violated, or do we assume that he did the issur and was chayev kareis until the age of 50 but from here on in is no longer chayev kareis? It seems to me that according to the Ra'avad, the milah done later in life doesn't have retroactive powers, it merely removes the issur kareis מכאן ולהבא, for the future, but not for the past. With this we can answer the question of the maharatz chiyus. If not for the passive nature of the violation, it would have been possible to bring a korban for this violation even after doing milah. Doing milah at the age of 50 wouldn't remove the violation until then, therefore if he violated b'shogeig he would be obligated to bring a korban even after getting milah done for the years of violation until then. It is only because milah is a passive violation and not similar to avoda zara, that there is no obligation to bring a korban.
A proof to this approach is the migdal oz who tries to justify the position of the Rambam by making him somewhat closer to the ra'avad. The migdal oz suggests that the Rambam agrees that he is chayev kareis immediately, but since he always has the ability to do milah and remove the kareis, the absolute violation can only be by dying without milah. He then continues to compare this to a לאו שניתק לעשה, where the la'av was violated but can be fixed. Based on this comparison, it is clear that doing milah at the age of 50 isn't a retroactive fix. Just as if one steals and then returns, he doesn't fix it retroactively and it only is a fix from the point of return, so too when he does milah it is not a retroactive fix and only serves to fix from that point forward.