Flat feet happen when the arches of the foot collapse or become loose. Arches can become weak as time goes by and make the ankle weaker. This causes discomfort along the upper body.
Flat feet can be a significant impediment to running and can cause a number of issues including discomfort in the heel and ankles and also a tendency of over-pronation.
Flat feet can come at any time and should be treated to avoid complications.