You know it's coming; it happens every month without fail. It's her period. Her period of what? Well, that depends on the girl. It could be crying. When a woman says that she is on her period, it means that she is menstruating. Menstruation is the technical term of what happens to a. The average age for a girl to get her first period in the US is 12, but the range of age is about 8 to 15 years old. Women usually have periods.