My Grandmother (on my Father's side) was 16 when she started her period. My Mother was 12 as well as me, so it's just genetics. Your period is caused from the lining in your uterus and as Sweetpea03 said, it is hormonal. It's caused when you release a unfertilized egg, the egg will try to attach itself to the lining of the uterus. If it is unfertilized, the body then knows to flush it out of your body which causes a period. Remember you can get Pregnant BEFORE your period even starts.
The sharp pains could mean you're starting to ovulate. Every month, about 2 weeks before I get my period, I'll get that aching pain, kind of like a cramp on either my left or right side. Depending on which side the egg is released. Also depending on if you're underweight or over weight, the hormones will be messed up and cause you to NOT have a period. Just look into this with a OBGYN doctor. You're close enough to start getting Pap Smears anyways, I was 15 when I started to get those.
I have 2 other brothers and MANY times was kicked in the vagina. Like I said up top, I started my period when I was 12. I think you're ok =) I'm a Mommy of 4 now~Good luck
there's most likely nothing wrong. you're period has to do with hormones, that's the reasons for why it hasn't started. talk to your mom about it.
Try talking to your doctor. He or she would be able to tell you more. You may also want to talk to your mom to find out when she had her first period. Maybe she was late as well.