Be willing to have a direct conversation about the age difference if you feel it’s become a real problem. Keep it short and direct, such as, “I’ve noticed that you comment a lot about my age. I respect you a lot and believe we can work together well and achieve our team’s goals. Can you let me know if there’s anything I can do differently to help us work well together?” You don’t have to do everything the person might ask for, but having this conversation shows that you want to make the relationship work. While you may never be beloved by this employee, it is your job as the manager to make your best attempt and be explicit about your intention to support her.