By adulthood, only one in seven people have flat feet. That still makes flat-footedness more common than many would expect. The cause is the same as it was when we were babies. The adult toe to heel arch is either absent or less than normal. The condition is not correctable.
Most feet are flexible.
Stand on your toes and an arch appears. If your feet are flat and you stand on your toes, the arch is missing or diminished. You can check the degree of foot flatness by standing with wet feet on a cement floor. The print of the foot will show a large contact with the surface. Flat feet lean inward when the person stands up straight.