The period of the most severe slippage is between 10-15 years old. No more slipping generally occurs after age 20. This is because of the positioning of the anatomical spine. It is more horizontal than in an adult spine, and thus allows more side-bending and rotation. This causes more stress on the lumbar disc and posterior column structures.
Sex: more common in women than men.
Anatomical: Some people are more prone to spondy because of the sagittal orientation of the facet joints, which can cause a greater amount of anterior flexion of the lumbar spine and thus predisposes the individual to a slippage.
Other factors may be lumbosacral angle, ligament laxity, previous pregnancy, and hormonal factors.