If the egg is not fertilized, the lining of the uterus begins to break down so it can be shed (along with the egg that was not fertilized). Your period – or the time of bleeding – happens when your body gets rid of the extra blood and tissue that's no longer needed. Then the cycle starts all over again. In the United States, the average age for a girl to get her first period is 12. This does not mean that all girls start at the same age. A girl can start her period anytime between the ages of 8 and 15. Most of the time, the first period starts about 2 years after breasts first start to develop.