Why haven't I started my period?

Hi everyone...I am 13 years old and I am scared because I haven't started my period yet. My mom and grandmother both started at 12 and my dad's family started at 12 too. I am 13 and 6 months and i have had the symptoms of being close to my first period for over 2 years...pubic hair, breast development, discharge, and basically the symptoms like i should get it any day now (yeah..this has been going on for awhile though). I just don't know why I haven't started. And I'm just scared. I mean i know some people do not start until way late like 16 but just the fact that my whole family started at 12 and i am already past 13 and a half is weird to me. So i know none of you can do anything about it... ;) but just tell me your experiences and what you know. K...that's about it!!! Thank you all so much!