I'm 20 years old my breast size is normal but nipples are very low.

Without seeing photos, it would be difficult to
assess your situation. I would recommend consulting a highly qualified, board
certified plastic surgeon to better asses your situation in person. You should
also do your research by looking at the surgeon’s before and after photos to
ensure they are in line with your expectations and desires. 

Thank you for your question.  Without a photograph it is difficult for me to make a recommendation to address your concern.  I recommend an in-person consultation with a board certified plastic surgeon to discuss your particular issue.  Best of luck.