Women tend to gain fat in their breasts and hips first, the whole body composition thing comes to mind.  But, if the person is genetically predisposed to have A cups there will only be so much breast tissue/muscle/collagen and such in that area.  They gain too much weight, it tries to store it where it's used to, but the platform collapses from not even support.  Hence saggy boobs.  Collagen tends to break down when you age, for older folks it happens.  Before the person is even 30 it's just sad.