The answer is Puberty.
The reproductive system remains inactive for several years after birth, however other organs begin to grow and develop. Usually between ages 8 and 14 for girls and 9 and 13 for boys development of reproductive systems begins. The hypothalamus of the brain secretes reproductive hormones which activates the reproductive system to prepare it for adult function. This surge is referred as the onset of puberty and leads to the beginning of sexual life.
