Babies are born with flat feet. Arches don't develop until walking begins and feet are not be fully formed until the age of 15 or so. Many adults first notice problems from flat feet in middle age.
Weight is a major cause, as it puts more pressure on the feet.
Losing weight frequently solves the problem. People with flat feet tend to overdo this action, throwing their natural gait out of kilter. This can lead to anything from shin splints to knee or back problems. It also puts abnormal strain on the big toe, possibly leading to bunions. If your flat feet begin to cause pain, see a doctor immediately.