Eating a plant-based diet has many advantages. There is a lower prevalence of overweight and obesity among vegetarian children and adults. Vegetarian diets are lower in saturated fat and cholesterol which lead to lower rates of heart disease. Plant-based diets also tend to be higher in fiber, complex carbohydrates, folic acid, vitamin C and E, carotenoids as well as other phytochemicals. This may help explain the lower rates of cancers and higher rates of cancer survival in vegetarians.
Vegetarians also have lower rates of diabetes. Other favorable side effects of consuming a low-fat, plant-based diet for children and adolescents are a decreased risk of constipation, acne and menstrual cramps.
A nice summary can be found on the Physician’s Committee for Responsible Website. Vegetarian Diets: Advantages for Children.