Flat feet are a condition where one’s feet do not have a normal arch when you’re standing. This can affect someone in different areas, including feeling pain when you do extensive physical activity.
The health term for flat feet is pes planus or fallen arches. It is normal in babies and usually disappears between the ages of 2 and 3 years old as the ligament and tendons in the foot and leg tighten. Having flat feet as a child is rarely serious, but it can last through adulthood.
The 2012 National Foot Health Assessment  showed that 8 percent of adults in the United States ages 21 and older have flat feet. Another 4 percent have fallen arches.
In some cases, flat feet are caused by injuries or illness, making it difficult to walk, run, and stand for hours.