Please tell me if it is normal to have your period at 9 years old?

Not quite: Unfortunately, young american girls are starting their periods earlier and earlier in the past 30 years due to better nutrition. Also, girls closer to the equator start sooner than girls in the northern climes such as the artic. I would check with the nine year olds pediatrician and make sure this early start is not due to what we call precocious puberty from an identifiable abnormal cause.

Yes: 9 is young but not unusually so. Girls are starting their periods at ever younger ages and i occasionally get reports as young as 8. We don't really have an explanation for this although i suspect it has something to do with diet changes and improved general health.