Periods are also known as menstrual cycles. The onset of menstrual cycles (menarche) occurs during the teenage years. Menstruation continues until a women is in her 50s and reaches menopause. The average age of menarche in the United States is 9 to 17 years of age, with a median age of 13.

Primary amenorrhea is a condition where a woman fails to start her menstrual cycles. If you have other signs of puberty, such as breast development or pubic hair, but fail to start your menses by the age of 16, you should see a physician. If you have no signs of puberty by age 14, you should see a physician.