People with lactose intolerance have trouble digesting lactose, the natural sugar found in dairy foods. This is because their bodies do not have enough of the enzyme lactase.
Lactose intolerance is not common in young children because most people are born with enough lactase. However, in some people, the amount of lactase in their body drops over their lifetime.
This is especially common in non-Caucasians and other populations that don't traditionally include dairy products in their diets.