According to the American Association of Orthodontists (AAO), "children [should] get their first check-up. . .at the first recognition of an orthodontic problem or at age 7.
Around that age, children have a mix of baby (primary) and permanent teeth. An examination at this age gives the child orthodontist a wealth of information. If a problem exists, or if one is developing, your orthodontist is able to advise you on which treatment is recommended, when it should begin, and estimate its length.
If your child is younger than 7, and you notice something that appears “off,” it’s not necessary to wait until your child turns 7 or until you get a recommendation from your dentist. You should take your child to an orthodontist the moment you notice an issue, regardless of age."