For most people who develop idiopathic scoliosis as adolescents, it doesn't cause any life-threatening problems, but it can make life difficult. As the curvature gets worse, the spinal bones(vertebrae) tend to rotate. This rotates the ribs, reducing the space in the rib cage and making it harder to breathe properly. This is only a problem for severe curvature, over 60 degrees. Severe curvature can also restrict movement. The earlier scoliosis begins and the higher up the back the curve is, the worse the curvature.
Attribution: BUPA