Can babies eat complementary food at 5 months old? Babies begin to have an increased demand for breast milk or milk powder when they are around 5 months old, which is really a big problem for new parents. If you want to add complementary foods at this time, you will have a question – can babies eat complementary foods when they are 5 months old? 1. It is reasonable for babies to start adding complementary foods when they are 5 months old. Five months is the period when babies gradually develop into the oral motor stage. During this period, babies may be particularly curious and like to try new things.