Menstrual cramps are contractions of the muscular wall of the uterus. The cramps are caused by a group of chemicals called prostiglandins. The type and quantity of protiglandins are influenced by the female hormones, estrogen and progesterone. The hormones, in turn, are produced by the immature eggs in the ovaries, responding to chemicals from the brain.
Teens often have more menstrual cramps and stronger menstrual cramps than older women. This is because their hormone levels are still somewhat irratic. When a baby girl is born, she already has her full supply of immature eggs. They are healthy and ready to go. But, of course, her brain, is still very immature. The exact mechanism that starts the first menstrual period isn't yet understood, but the maturing of the brain gets the whole process going. Since the brain continues to mature, cycles become more regular, and cramps often decrease, as a woman moves through and beyond her teen years.