Most girls get their first period when they are around age 12 or 13, but each girl is different. Here's how it works: about 2 years before your period comes you start to get boobs and while your boobs are growing, you start to get hair down there. You know you are close to getting your period when your boobs are larger but the nipple and skin around it sits like a little mound on top of the breast. It is around this same time that you will grow in height like crazy and within 6 months or so your first period will come. Woo Hoo! Some girls start to get boobs at age 7 and will have their period around age 9 while other girls don't start getting boobs until they are 13 and those girls won't get their period until they are 15 or so. If you haven't started to get boobs by the time you are 14 or your period hasn't started within 2½ years from the time you started getting boobs, you should check it out with your healthcare professional.
Every girl wants to know the answer to this question especially once they learn what a "period" is. Since every girl is different, the ages at which they start their periods is different. Just because you haven't gotten yours doesn't mean that your body isn't getting ready for it. I started very young - at age 11 - but my sister started hers at age 15. Ask your mother or an aunt or a grandmother if they can tell you when they started their period. You may find that your body is following in that same pattern and you'll start a little later.
Consider yourself lucky, you don't have to deal with it yet! No worries though, there isn't a set age when girls start their periods, it changes between every different person - some get it earlier than others and some, later. It'll come in due time. In the meantime, take this time to prepare yourself, make sure you have tampons or pads on hand for when it decides to start and a friend who'll know what you're going through. If you don't get your period before you're 16, you should talk to your healthcare professional.