Children's feet normally keep growing until the growth plates of their bones close. This takes place at age 13 to 15 in girls and age 15 to 17 in boys. Feet tend to spread with age, so an older person may require a larger shoe than when he was younger.
Aside from age, pregnancy is the most likely reason for a change in shoe size. This is because the arch of the foot tends to flatten during pregnancy, causing the foot to grow longer by 2 to 10 millimeters. The change is often permanent and may be related to why women are at higher risk than men for chronic pain or arthritis in their feet, legs and spines.