My daughter is 15 and has extremely large breasts with curved posture, when is it recommended to get a reduction?

I'm sorry to hear about the physical and (probable) psychosocial problems your daughter's disproportionately large breasts are causing her.  I am glad however that she has your support and understanding.
It is probable that she is dealing with juvenile breast hypertrophy along with the physical and psychosocial consequences of this diagnosis. In other words, the breasts are too large for her frame causing  both physical and psychological distress.
Although it may be slightly too early to undergo this procedure, 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.    Again, it is hopeful that she has her support as you both do the homework.
Unsatisfactory scarring is  one of the potential complications. Make sure she 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. Careful selection of your plastic surgeon should help alleviate some of your safety concerns; from this decision, will stem other important safety considerations such as selection of anesthesiologist and surgery facility as well. Ultimately, your daughter should be able to undergo this procedure with minimal risk to her life/health.
I hope this, and the attached link, helps.

Excellent question!  The insurance companies in NY will no longer authorize surgery for girls under age 18.  As an E cup, she will be covered when she reaches that milestone.  Conservative medical treatment is required--many carriers insist on wasting 6 months of your time and all those co-pays on physical therapy or chiropractic.  It is a good idea to move forward with laying the foundation for approval in the next year or two.In terms of risks of surgery, they are extremely low.  Anesthesia is performed by board certified anesthesiologists in an accredited hospital setting.  Given the effects on her back, chronic pain and issues with self esteem, surgery will be major life changer.

If your daughter has reached adult size and is really not growing any more, it may be reasonable to perform a breast reduction. Best for her to see someone in person for evaluation. Good luck!