A flat foot is 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.
Some children have flexible flatfoot, in which the arch is visible when the child is sitting or standing on tiptoes, but disappears when the child stands. Most children outgrow flexible flatfoot without problems.
Arches can also fall over time. Years of wear and tear can weaken the tendon that runs along the inside of your ankle and helps support your arch.