If the sides of teeth are touching together, toothbrush bristles cannot reach between to clean. Any spots where teeth are touching should be flossed daily. Baby teeth usually begin to touch around age three, especially between the back molars. Dr. Khoury recommends practicing your flossing techniques with your child as young as two years. In the beginning, a parent will need to floss. As a child gets older, parents can supervise and allow their child to floss. This skill is usually mastered by middle school.