I'm a 15 year old and my breast are already sagging, what can I do?

Each case like yours has to be considered individually.  If your breasts are extremely large and uncomfortable, performing a breast reduction at your age is not unreasonable.  However, you would need parental permission, so you should discuss the issue with your parents.

Thank you very much for sharing your concerns with us, about your breast size.After having analyzed all the information sended to us, i can realize that you have a very large and heavy breasts for your age, this produces discomfort, and in the future may cause osteo-articular damage in the back and shoulders, and skin problems (mainly under the breasts).For these reasons we should consider performing a breast reduction surgery on you.As part of the preoperative preparation i recommend an endocrine and mastologist evaluation, who should do the studies (Ex .: mammary sonography) and relevant blood tests, to ensure that the excessive growth of your breasts is normal, and is not associated with any hormonal disorder or other cause.Finally I remind you that you're a teenager, and, for any type of procedure we need a legal consent of your parents or responsables.Respectfully,Dr. Emmanuel Mallol Cotes.- 

Breast reduction and lift can be performed to correct large sagging breasts. This can be performed at your age if certain criteria are met. The best thing to do would be to have a consultation with a board certified plastic surgeon to see if you are a candidate for this procedure now or possibly in the future. This will give you the opportunity to find out more about it, including what to expect and possible risks.