Everyone grows and develops differently. So, usually girls have their periods around the age of 11 to 13. A girl’s breasts usually start to develop around the age of 10, followed by the appearance of pubic hair and armpit hair. In some cases, menstruation occurs first, followed by armpit hair, and in some cases, armpit hair occurs first, followed by menstruation. If a girl’s breasts develop before the age of 8, this is not normal and is usually precocious puberty, so you can go to the hospital for a systematic examination.