Swayback, also known as lumbar lordosis, is common in children and young teens. Some don't develop the abdominal muscles fully to hold the spine in an upright position until much later. Even so, good posture can help reduce muscle fatigue and prevent injuries.
Research shows that women do have more lumbar curvature than men. Whether or not increased lordosis is linked with low back problems is unclear.
Physical therapists at the Washington University in St. Louis are studying the relationship between lumbar curvature and low back pain. So far they report that people with back pain have about the same amount of curvature as people without back pain.
The difference occurs in certain types of low back pain. Women have larger lumbar curves. Women are more likely to have problems with rotation and extension when they have low back pain. Perhaps there is a link here but more study is needed to show what it is.