The beauty about Mother Nature is that no two people are alike, which means that a person begins their period when their body is ready. On average, most people will start their periods when they are 12 years of age, but they can start, as early as 8 years of age. And on the flip, as late as 17 years of age.
Signs that your period is on its way are if the person has grown underarm and pubic hair. Typically, a person starts their period about two years after their breasts start growing.
See your doctor if your periods have not started by age 15, or 13 if you do not have any signs of puberty. Your doctor may suggest some tests to check possible reasons for delayed periods.