It is normal to have a flat foot if you are a baby or a toddler because it takes a bit of aging before the arch becomes fully developed. Usually the arches form in childhood but, in some cases, this never happens and the foot or feet remain flat.
It is possible to have a flat foot that is flexible. This means that the child has a visible arch as long as the child is sitting down or standing on his or her tiptoes. As soon as the child stands directly on the floor, however, the arch disappears. As the child grows, the ligaments that create the arch become stronger and the flat foot disappears, leaving behind a normal arch.
Adults can have flat feet even if they once had normal feet. People who are on their feet a lot will cause the ligaments that create the arch to weaken, causing the foot or feet to become flatter.