Why does kwashiorkor often occur among children six months to eighteen months old?

Nutritional reserves. Primarily seen in african countries, kwashiorkor is a protein-calorie deficiency, seen in starvation. The younger children have less muscle, less reserve and therefore more susceptible, particularly upon transition away from nursing. Some would argue that kwashiorkor should be a term applicable to starving adults as well, but protein deficiency edema in starving adults may have other causes.