Sagging is a normal part of how the body changes. And while some may say that saggy breasts are the result of weight gain and loss and pregnancy, it’s really breast involution(when the milk producing glands shrink and start to shut down) coupled with these body changes that causes the breasts to drop.
Age has something to do with sagging breasts, but is not as much of a factor as pregnancy and genetics are.
Breasts do not have muscle, but rather suspensory ligaments and tissue. When the cooper’s ligaments, which suspend the breast tissue, get stretched out, they can’t recoil and support is lost. And if the skin gets stretched, the dermis can be damaged (and unable to contract back), which is why stretch marks appear.
Losing weight can also cause the breasts to sag since volume is lost. There is a lot of fat in the breast and if you tend to lose weight in your breasts, you will probably lose volume too. It is also believed that the larger the breasts, the more quickly and noticeable your sagging will be.