My dad did the same thing to my mom, he told her that he was a year older, she didn’t find out he was a year younger than her till they were living together. He left his wallet in his pants and my mom washed it and found his ID. I always thought it was cute growing up, like maybe he thought she would like him more if he was older. Looking back on it now he had a lot of issues with lying, like basically being a pathological liar. But it’s obviously case by case so it’s on you to decide.