There is no federal law setting a legal age minimum before children can walk to school alone. For a law specific to your state, contact the Safe Routes to School Coordinator local with in your state's Department of Transportation. You may also want to contact the school district or school where the child attends to determine if they have a policy that prohibits children under a certain age from walking to school alone.
According to the National Highway Traffic Safe Administration (NHTSA) children, even in the same grade, vary in their readiness to handle traffic situations, such as choosing a safe time to cross a street. In general, children are not ready to cross a street alone until age 10. Ideally parents are a central figure in their children's safety education. Parents have the best opportunities to effectively assess their individual child's skills and teach safe behavior in the course of daily life so they should be encouraged to participate in their child's safety education. It is less a matter of chronological age, and more about whether children have demonstrated that they can safely walk and cross streets independently. For additional information about when children are ready to walk alone, see our Teaching Children to Walk Safely as They Grow and Develop resource.