According to policies at my institution, instructors (graduate TAs) cannot be involved romantically with students when they're teaching the students in their TA capacity but after the course ends, there's nothing wrong with it in terms of policy. If you think about it, most graduate students are in their mid- to late-twenties and if you're likely around 20 as well - the age difference isn't really that big. However, since this is a pre-med instructor, you might run into problems in the future if, say, you want a letter from the professor and he or she needs to ask the TAs about your performance, etc. This creates an obvious conflict of interest.