The Canadian and American Associations of Orthodontists both recommend a first orthodontic check at about age seven. By then a child has a mix of baby and adult teeth, and problems that are simple to influence now are visible for the first time.
The check is not a sales appointment. Most seven-year-olds we see need nothing at all beyond a review in a year or two, and we will say so. What the visit buys is timing: knowing which children need something now, and which are better left until all the permanent teeth are through.
For the minority who do need early treatment, the opportunity is real and it closes. While a child is still growing, an expander or a partial course of braces can widen a narrow arch, correct a crossbite, create room for crowded teeth and guide the jaws into a better relationship. Wait until growth is finished and the same corrections may need extractions, or surgery.