Even though 12 is the average age for your periods to start, the normal range is between 11 and 16 years old. The age you start depends a lot on your genetics and also your body mass index (BMI). Are you quite slim or do you play a lot of sport? In any case, you shouldn’t worry until you are at least 16. In fact, enjoy your period free time! Breast size is mainly determined by your genetics and also your nutrition (ie. a healthy balanced diet). Your periods usually start about a year after the start of breast development but this can vary. It usually takes 3-5 years to complete.
Generally having periods doesn’t hurt. However some girls experience PMS (premenstrual syndrome) in the couple of days leading into a period and/or the first day or two of the period. This can include some abdominal cramping and discomfort. But not everyone experiences this and they may not have it every month.