Question

I seem to remember that when I was in yeshiva I asked this question and didn’t find an answer, so I’ll ask it here. A. It’s known that an uncircumcised man incurs karet—does that begin from bar mitzvah age? B. After all, in principle he can “fix” the situation, so does the simple meaning mean that every single second he remains in a state of prohibition, or does he transgress only when he dies and it turns out retroactively that he died because of the karet (according to the view that there is providence)? And if he did in fact correct it, is the karet erased for him?