Totally normal, don't worry yourself over it. Every girl is different - as long as you start by the time you are about 16, there is nothing to be concerned about. If you are very athletic or very thin, either of those could affect your ability to store estrogen
. You need estrogen to menstruate.
If you STILL don't have a period by 16 or so, then you'll want to see a doctor to find out if you have a normal reproductive anatomy and normal hormone levels.
One last thing: don't stress
about it. Anxiety
over menstrual periods can actually goof up your cycle and delay menstruation