Turnout faults, are not really faults as such. The human body is not designed to function with 90 degree or over turnout. The faults that arise from turnout, are usually from forcing the legs and feet to rotate further than anatomically possible for that particular body. A very small proportion of people have extremely supple hip sockets that allow for maximum rotation. Unfortunately the few make the 'ideal' turnout what everyone else is aiming for.
This brings about many complications in the hip socket, knees, and feet. The most common is 'rolling' from forcing turnout so the feet roll inward. After a time this then effects the knees. Pain in the knees and in the feet can result from this. Hip socket pain is common also as turnout is actually from the hip socket, not the feet or the knees.
Ideally, gentle stretching from when one is very young is the best way to develop turnout. This is why many dancers start ballet at a young age.