There are so many factors that will affect the size and shape of your breasts as you develop and get older. Puberty, hormones, and weight will fluctuate as you grow. Your breasts will change. When you are closer to 18-20 years old you will have a better idea of how your breasts are going to be. But even after that they could change (with pregnancies, weight fluctuations, etc.). Saggy breasts can be caused of lack of volume after weight loss or simply by genetics. Having a consult with a surgeon when you are ready can help determine if you need a breast lift. Sometimes using an implant can give you more volume which results in a lifting effect, whereas if the skin is lax or loose, you could need to have an actual breast lift surgery. Have a great day!