Note that harnesses are not made to help toddlers learn how to walk, they are for kids who are already walking.  A safety harness for toddlers is recommended as soon as your child can walk on his or her own.  You can gradually reduce its use or stop it when your child has increased his skills and now has the ability to stay close and listen to instructions.  For kids with special needs, you may use the harness for longer periods if they need more time to learn the skills needed to walk safely.