Spondylolisthesis is a condition in which one vertebra slips forward on the one below it. In children, spondylolisthesis may occur as the result of a birth defect that affects the back of the spine or be caused by stress fractures within the back part of the spine. Spondylolisthesis is the most common cause of low back pain in adolescent athletes. In older people, the most common cause is degeneration of the discs between the vertebrae. With aging, the discs lose moisture, dry out and flatten, bringing the bones on either side closer together to the point where one slips forward on the other.