Will Tricare Prime cover my breast reduction a couple months before my 21st birthday, when I get taken off my parents insurance?

I am sorry to hear about the problems your disproportionately large breasts have caused.  Based on your description, you sound like you may be an excellent candidate for breast reduction surgery; of course, much more information is necessary to provide you with definitive advice.

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. Also, for many insurance companies, the closer you are to your long-term ideal weight, the better.
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. In regards to breast size desired, also communicate carefully. In my practice the use of goal pictures are helpful in this regard; a discussion of cup size can be confusing and imprecise. Therefore, I would not suggest that you communicate your goals and/or base your satisfaction with the outcome of surgery on achieving a specific cup size. For example, a "C cup" may mean different things to different people. I also find that the use of pictures is more helpful than the words “natural” or "proportionate” etc., which can (again) mean different things to different people. Many of my patients choose to have enough breast tissue removed to help alleviate symptoms while retaining enough breast tissue to remain proportionate to the remainder of their torso. Again, preoperative communication will be critical. I hope this, and the attached link, helps. Best wishes.

It is important to start the pre-approval process now.  It can take time to get pre-authorization.You only have about six months but this seems doable.See a PS for an exam soon.