I am 18 years old and haven't menstruated since I turned 17. I started when I was 13 and was pretty regular from a year after that. I have had clinical depression for over 2 years and have been diagnosed with PTSD, but I do not think this could have caused anything but a couple missed periods. I'm healthy, of appropriate weight and have not been on the pill. I just can't think of any good explanations.