Can You Have Sex If You Haven't Started Your Periods?

As girls usually have started their periods by the time they are of the age of consent, i would say they should not have sex before then. A girl having a period is only at the beginning of the changes that her body is making before she really is ready to have sex

That would depend upon your age and circumstances. If you are only 13 and haven't started your periods I would advise against it because you are still very young and pregnancy is still possible. A quick look through any human biology book/family planning book will explain the reasons for this or alternatively you could discuss it with a trusted adult. Also, at thirteen you are way to young and it might be better to wait a few more years until your body matures - a willingness to have sex ISN'T an indication of love and affection and it would be better to keep your self respect. If you are seventeen or eighteen, you are in a stable relationship and have had investigations to determine that there is a medical reason for your lack of periods then that is totally different - but again, using precautions would be sensible unless you wanted to risk an unplanned pregnancy.
Hope this helps - Kxx