People can start their period at a wide range of ages. Even if your mother or sister got theirs at younger age you can get yours later. The normal age range I think is 9-15 yrs. Periods starting later than that is usually because of eating disorders. You can check a bmi calculator to see if you are at a healthy weight. I got my period at age twelve, but everybody is different, and other things including lifestyle, stress, and hormones affect your period as well.
Come to think of it maybe that isn't what you meant by your question at all. You've already started your period? Ok, well I have amenorrhea which causes my period to sometimes take a break for as long as four months. I am also recovering from an eating disorder, so although I am eating better now my hormones are still out of wack. If your periods are usually regular, I just suggest you talk to your doctor. Stress is a big thing. If you've been having more stress lately, that could be the cause. You can't get pregnant without having sex- just in case you didn't know.
