You can schedule an appointment with an orthodontist at any time to discuss your orthodontic needs and establish a treatment plan. Likewise, if your dentist refers you, you don't need to choose that orthodontist. You're free to see who you think can do the best job for you. Dentists often refer patients who they think should receive orthodontic treatment.
However, orthodontists generally do not require a referral from a dentist to diagnose and treat a malocclusion. If you think that you or a member of your family will need a realignment or correction, you can call an orthodontist and schedule an initial consultation. In many cases, the orthodontist will do it for free. Many orthodontists say that a child should see an orthodontist before the age of 7 so that any problems can be detected in time.
That doesn't mean a child will get braces fitted right away. However, the orthodontist will know what problems exist and will be able to choose the best time to start treatment.