It depends on the kid.

If your general dentist thinks that your son needs orthodontic treatment, you will be referred to an orthodontist and he will tell you what level of treatment is needed, when to start, how long it will take and how much, etc. Each kid's teeth and physical development is different. Generally speaking, you want to start treatment before the physical growth spurt starts. My son is 10 years old. But he is dental advanced. We've been shopped around already and everybody agreed that he should start treatment sooner than later. I asked the orthodontists and they told me that this was a very standardized profession. If we do need to switch orthodontist due to job change, another orthodontist at a different location can very easily pick up the rest of the treatment. For my son, the entire treatment will be around 2 years with retention braces until he is 18.