15 years old and I'm a K cup and continuing to grow - Am I at a good age for breast reduction?

Breast reductions can be done in minors as long as they have
parental consent. If you are a size K
and having neck, back and shoulder pain then you are a good candidate. I recommend a new technique called The
Ultimate Breast ReductionTM.
This technique avoids the ugly vertical scars of the traditional anchor
incision, maintains nipple sensation and the ability to breast feed. Best Wishes,Gary Horndeski, M.D.

I'm sorry to hear about the physical and (probable) psychosocial problems your disproportionately large breasts are causing you. 
It is probable that you are dealing with juvenile breast hypertrophy along with the physical and psychosocial consequences of this diagnosis. In other words, the breasts are too large for your frame causing  both physical and psychological distress.
  In my practice, timing of breast reduction surgery is determined on a case-by-case basis. It is 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.  Hopefully you have the support of your parents as you do your homework.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).
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).

Sometimes breast  reduction surgery is covered through health insurance. The best way to obtain insurance coverage for breast reduction surgery involves some “hoops” to jump through. 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.
 When the time is right,  seek consultation with well-trained/experienced board-certified plastic surgeons.
I hope this,  and the attached link, helps.

I think that at the age of 15 you could do a breast reduction with your parents consent. You should see your local Plastic Surgeon for a consultation to see what they recommend.