Im only 17 and my breasts are saggy, what's causing this and how can i fix it?, i wear a bra (padded) most of the time.

Short ligament: Most likely a short lower ligament causing the sagging and would need a surgical lift to resolve this long term and can use a push up bra to help in clothes. I hope this helps.

Body mass index?: If you're overweight, weight reduction will help. Otherwise, I'm doubtful that there's anything you can do short of plastic surgery. A good bra makes sense (as you're doing).