Orthodontic treatment (braces) before the age of seven is something that we normally don’t look to do. Creating any significant improvement before that age is usually not practical because there just aren’t enough permanent teeth present. Preventing the need for more extensive treatment, such as jaw surgery and extractions, by catching and treating them early, is why we, as Orthodontists, do like to begin seeing children at about that age.
You just cannot know which children need treatment unless you are able to check them, so even though most will not, they will still need to be screened, and doing so about the age of seven is what we (and the AAO) recommend.
So that any needed treatment will not take any longer than is absolutely necessary for those who do need it, we always try to be as efficient as possible in timing the start of such treatment. By putting braces on too soon, however, with wires that move faster and faster these days, a patient could end up wearing the braces for a longer period of time, instead of getting them off sooner.