I am a 20 year-old (virgin) female, and for the last three years, I have been having very irregular periods. I started menstruation at the age of 10 (which is about average for the females in my family--I even have a cousin who started when she was 8), and up until I was seventeen, my periods were VERY regular. I had a period exactly every 21 days, and it would last no longer than four days. In fact, I could always determine the EXACT day each month when I would start and stop.