Supplementary foods can be added slowly after 4-6 months of age, following the principles of small to large, thin to thick, fine to coarse, and one to many. At 6 months of age, solid foods are added, and the baby gradually transitions from eating only breast milk or formula to eating a variety of foods like an adult at 2 years of age, which is an important stage for him to establish healthy eating habits. Diagram of food transition for infants