Most girls start to menstruate between ages 10 and 15 years. The average age is 12, but every girl's body has its own schedule. Although there's no one right age for a girl to get her period, there are some clues that menstruation will start soon. Typically, a girl gets her period about 2 years after her breasts start to develop. Another sign is vaginal discharge fluid (sort of like mucus) that a girl might see or feel on her underwear.