Complementary feeding should be started at 6 months of age because by this time, breast milk alone no longer meets the infant's increasing nutritional and energy requirements. At 6 months, the infant's digestive system is sufficiently mature to handle solid foods, and introduction of complementary foods supports optimal growth and prevents nutrient deficiencies, especially iron. Early introduction before 6 months can increase risks of infections and allergies, while delayed introduction beyond 6 months can lead to malnutrition and growth faltering. Therefore, initiating complementary feeding at 6 months is critical for balanced nutrition and healthy development.

Reference: Nelson Textbook of Pediatrics, Kliegman RM, Edition 21.