I was the same way in 7th grade. All my friends had started and I still had not. I didn't start until 9th grade. Every girl is different. I always felt left out, until I actually started, and I realized I wasn't really missing out like I thought I was. It doesn't mean you aren't as mature as them or anything else. We are just the type of girls whose body took a little longer.. we are unique :)
We all have our own timeline for our bodies. We have a specific number of ova(eggs), so our periods begin and end with the first and last eggs. When the time comes, it will be the right time for you. If you are really worried about this, see a doctor.