There is no set age when children need orthodontics. It depends on a child's individual needs. For example, kids with cleft palates get orthodontic appliances before their first teeth come in.
Other kids may benefit from starting treatment as early as age 6 or 7, even if they still have some baby teeth. The goal of early treatment is to prevent further problems later on.
Most kids who need early orthodontics will still need braces or additional work later, but it may be significantly less.