Summarize this article:

It’s normal for kids to have flat feet at least until the age of 5 years (and sometimes as late as 10 years) because it takes time for the bones, ligaments and tendons in the underside of the foot to form a supportive arch. As such, don't panic if your child has flat feet, especially if they don't seem to be causing pain or problems with walking or running — they will likely grow out of it, so no need to seek treatment and try to fix it.  Do the flat surface test to determine flat feet. Moisten your feet and step onto a dry surface that shows your footprint. If the entire surface of your foot can be discerned from the print, then you have flat feet. A person with normal arches has a crescent of negative space on the inside (medial) part of their footprint due to lack of contact with the surface. Flat feet in children rarely cause pain. A tight Achilles tendon from birth (congenital) puts too much pressure on the front 3/4 of the foot, preventing a normal springy arch from forming. The Achilles tendon connects the calf muscle to the heel. When it's too tight it causes the heel to lift off the ground prematurely during each step while walking, causing tension and pain underneath the foot. In this instance, the foot becomes flat while standing, but remains flexible while not weight bearing.  The main treatment options for flexible flat feet with a congenitally short Achilles tendon are either an aggressive regimen of stretching or surgery, which are explained in more detail below. In addition to arch and heel pain, other common symptoms of flat feet include: calf, knee and/or back pain, swelling around the ankles, trouble standing on tiptoes, difficulty jumping high or running fast. A rigid, inflexible flat foot remains without an arch whether weight bearing or not. It's considered a "true" flat foot within medicine because the shape of the underneath of the foot remains unchanged at all times, regardless of activity. This type of flat foot is typically caused by bone malformations, deformity or fusions that prevent the arch from forming during childhood. As such, this type of flat foot can be present from birth, or acquired in adulthood due to a foot injury or disease, such as osteoporosis or inflammatory arthritis.  A rigid flat foot often creates more symptoms because the entire biomechanics of the foot are altered. Rigid flat feet are most resistant to accommodative therapy such as shoe inserts, orthotics and physiotherapy. Another kind of flat feet is often referred to as adult-acquired, but it is usually due to overstretching / overuse / damage to the posterior tibial tendon, which runs from the calf muscle along the inside of the ankle and ends within the arch. The tendon is the most important soft tissue of the arch because it provides the most support. The most common cause of posterior tibial tendon overstretching is having to support too much weight (obesity) for too long, particularly if unsupportive footwear are typically worn.  Flat feet is not always bilateral — it can occur in only one foot, especially after suffering a fractured ankle or foot. Adult-acquired flat feet often respond to accommodative therapy, but losing weight is often the key to fixing the problem.

Summary:
Flat feet in children is normal. Tight tendons can cause flat feet. Rigid, flat feet are caused by bone deformity. Adult-acquired flat feet are often due to obesity.