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 developif a girl has not had her first period by age 15 or if it has been more than 2 to 3 years since breast growth started she should see a doctorhis 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