Why do my breasts fall to the side?

A breast lift may help but I would not recommend this for a 16 year old. This is a decision with permenant consequences that most 40 year olds have problems dealing with let alone a 16 year old 

If at 16 you are fully developed and get permission from your parents, then it might be best to go for a consultation.  A lift and reduction may be a reasonable approach.

Hi Lele. Some young women go through a process called juvenile breast hypertrophy. As in your case, the breasts grow very large very quickly. Many patients who go through this benefit from a breast reduction. In this procedure a large amount of breast tissue is removed to decrease the size and weight of the breasts. At the same time the nipples are lifted up into a better position on the new, smaller breasts. This makes patients feel and look better. Your best bet is to find a reputable board certified plastic surgeon in your area and schedule a consultation to learn about breast reduction surgery and breast lift surgery. Good luck. Dr J

Sounds like you have large breasts that have a ptosis (droop) problem.    This can be corrected with a vertical mastopexy-reduction.   Seek a consultation with a plastic surgeon and sort out your options.  My Best,  Dr C