Concerned

About a month ago I met this wonderful girl. We didn`t get to spend much time together, but we just clicked...the feelings were there. But there are a couple of problems. First off, she lives 1300 miles away, and second I am 21 and she is 16. I care for her deeply, but I dont know if a long-distance relationship with a girl that age will last. She seems very mature when I met her. I went to visit her last week and things weren`t what I thought. What should I do?