Level 1: Why should we add complementary foods to 5-month-old babies? After the baby is born, breast milk or formula milk has been the baby\’s main food. However, as babies grow, they need more nutrients to support physical development and growth. At this time, you need to add complementary foods. Around 5 months old, your baby\’s digestion has developed to the point where he can accept other foods, so it is important to add complementary foods.