Every girl starts her period at a different time, but the average age is around 12. This can vary a lot – some girls start around age 8, others not until their later teens. Periods tend to come along after other signs of puberty (like breasts developing and underarm/pubic hair appearing) have begun to show.
At first, while the adolescent female body gears up for regular periods, cycles can be very changeable. Some girls may have regular, predictable, 28 - day cycles right from the start, but many more find that periods come at odd intervals, sometimes spaced far apart, sometimes close together.
If you are worried about your menstrual cycle it can help to talk to a doctor about it. It can also help to know what’s “normal” and what isn’t: