I am sorry to say it is nearly impossible to say for sure! Most girls will start between 11-14 but it is not uncommon to start from as young as 8 or 9 too. Some girls won't start till 16 or 17 either. It really depends on your body and your unique hormones. If you feel it is getting a little late and you are worried have a chat to your doctor.
Want to know the clues? Usually they don't start until a few years after your breasts have begun to grow or your pubic hair starts appearing. Asking mum when she started can also give you a good indication. It is often likely you will start within a year or so of when she did.
Another dead giveaway is when you notice a white or creamy coloured discharge or stains on the inside of your knickers. Sounds pretty yuck hey but its nothing to worry about and totally normal. It happens when the walls of your vagina start producing a fluid to keep them healthy. It usually means your period could start a few months after this occurs. If you worry about staining your knickers or the wetness bothers you try our leak proof knickers ! Mynickerbot makes leak proof knickers for tweens in youth sizes from size 8 - 14.
One day down the track you will probably least expecting it you will go to the loo and see some red or brownish smears in your knickers then you will know she has arrived.
We have your back. MNB.