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 school coordinator for that state's Department of Transportation. You may also want to contact the child’s school or district to determine if either has 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.