Bow legs in adults are usually caused by knee arthritis, or wearing away of the cartilage of the knee joint. If the cartilage is worn away unevenly (i.e. more from the inside of the knee), then the knees may become bowlegged.
Bow legs in children are an entirely different issue that are unrelated to arthritis. Bow legs in children are a normal part of the growth process. In some cases, bowleggedness in children can be related to an underlying problem called Blount's Disease.