In the fairness of this being a discussion (originally) in the impact of bralessness on Grokette, she most likely would have breastfed children. We don't know for how many children or for how long, however. Diet definitely plays into the lives of the modern tribal folks referenced. I am sure they are not always in times of plenty. Any (not just significant) weight gain and loss will also affect the shape and "recovery" of the skin.
It is my understanding that a "properly fitting" bra will help prevent all the back problems and such that people tout. I would imagine that a well-fitted "minimal" (read: support as needed, not the padded monstrosities that make women feel inadequate such as exist today) bra combined with occasional free-and-loose would make for the healthiest of breasts in this modern age.