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. When did your mum start her periods? 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). Breast development usually starts about one year before your period starts and is one of the earliest signs of puberty. It usually takes 3-5 years to complete. So if you haven’t started your period yet, your breasts still have a lot of developing to do. As hard as it is to not compare yourself to other girls your age, remember that every girl develops in her own time.