I am 18 years old and I want breast reduction but not sure if my doctor would approve?

It is difficult to tell in your case without photos or a description of your physical symptoms.   Insurance approval depends on multiple factors.  These include the patient's breast size compared to their height and weight, the plastic surgeons estimate in terms of breast tissue weight to be removed, the extent of the patient's symptoms and physical limitations as they relate to her breasts and whether the patient has sought conservative treatment for their symptoms such a s chiropractic care or physical therapy.  Unfortunately every insurance companies requirements are different.  I recommend consultation with a board certified plastic surgeon who can better evaluate your situation and write a letter of preauthorization to your insurance company. 

Hard to tell whether you would be a good candidate without in-person evaluation or viewing photographs.In my practice, timing of breast reduction surgery is determined on a case-by-case basis. It is certainly not too early to do your homework and learn as much as possible about breast reduction surgery and the potential risks and complications associated with the procedure. 
It will be important that you learn about the potential risks/complications associated with breast reduction surgery. Unsatisfactory scarring is one of the potential complications. Make sure you also understands that further surgery may be necessary in the future (for example if the breasts were to grow in size again).
Some of the risks/potential complications associated with breast reduction surgery include: infection, bleeding, seroma, hematoma, wound healing problems, abnormal scarring ( hypertrophic or keloid), loss of sensation, inability to breast-feed, breast asymmetry, necrosis of tissue or skin, unsatisfactory cosmetic results, unpredictability of exact cup size postoperatively, recurrence of the breast hypertrophy, and the potential for further surgery. other risks related to surgery in general include deep venous thrombosis (clots), pulmonary embolism, pneumonia and even death.
Fortunately, the vast majority of patients who undergo this procedure by well experienced board-certified plastic surgeons and board certified anesthesiologists do very well and complications tend to be relatively minor and treatable. The severe complications are rare.
On the other hand, breast reduction surgery is one of the most patient pleasing operations we perform and I think that for the right teenager (enough symptoms) it may be an excellent option (regardless of the age).
  When the time is right, seek consultation with well-trained/experienced board-certified plastic surgeons. Ask to see lots of examples of his/her work achieving the types of outcomes you would be pleased with.Best to check directly with your insurance company to see whether breast reduction surgery is a “covered” benefit with your specific policy. Then, given that every insurance company has different “criteria”, best to identify exactly what your specific insurance company considers criteria for approval. Generally, the more documentation you have (for example, from your primary care doctor, physical therapist, chiropractor etc.) the better when it comes to obtaining insurance “authorization” for the procedure. 
This documentation and letter/pictures from your plastic surgeon will help you obtain authorization. Keep in mind, as you work towards your goals breast reduction surgery is one of the most patient pleasing operations we perform. Also, prepare to be persistent when dealing with health insurance companies. 
I hope this helps. Best wishes.