Not all feet are the same
Flat feet are normal in infants and toddlers, because the foot's arch hasn't yet developed. Most people's arches develop throughout childhood, but some people never develop arches. This is a normal variation in foot type, and people without arches may or may not have problems.
Lax ligaments can cause flatfeet, as can conditions present at birth (congenital) that affect the foot.
Arches can also fall over time. Years of wear and tear can weaken the posterior tibial tendon, which runs along the inside of your ankle, from above your ankle to your arch. The posterior tibial tendon is the main support structure for the arch. An overload to this tendon can cause inflammation of the tendon (Tendonitis) and even tearing of the tendon. Once the tendon is damaged the foot's arch loses support and can flatten.
You may lose support in your arches due to: