1. Why should we add complementary foods to 9-month-old babies? After your baby is born, breast milk or formula is his main source of nutrition. But over time, babies need more nutrients to support their growth and development. Therefore, you need to start adding complementary foods when your baby is about 6 months old. And when babies reach 9 months, they need more solid food to meet their nutritional needs. 2. What complementary foods are suitable for 9-month-old babies? 1. Cereals: You can add some brown rice porridge, millet porridge and other cereals to your 9-month-old baby.