Most girls will get their period about 2-3 years after they start to develop breasts (on average) and some will not get their period until later. The average age for girls to start breast development is 10 and the average age for periods is 12 to 12 1/2. Girls who start breast development very early will often take longer (3 to 4 years after starting breast development) to get their periods. There are other factors too. Girls who are active in sports or are very thin may not get their period until they are older. Losing weight while you are growing can also delay your period from starting. If you don’t get your period by the time you are 15 or it’s more than three years since you started to develop breasts, (whichever comes first), you should make an appointment with your health care provider.