If it were young Law, I think neither would really care and Cora would go along with it for the sake of convenience. After all, it's easier to let people think he's her son rather than some random kid, or worse, her boyfriend lol. I think adult Law would absolutely not take kindly to the assumption, and loudly correct them (how dare someone call his Cora-san old??). Again, Cora wouldn't care that much but she would be slightly embarrassed by Law defending her. What's funny is I think they both have very short-tempers, but Cora has the benefit of maturity on her side by that time.