I honestly don't think wearing a bra prevents the effect of gravity in the long run. Gravity wins, regardless. When I was younger and thinner, I went bra-less because I had less boobage. These days, as a C cup, I never go bra-less. Now I'm 36, and they're no more- or less-saggier than any other non-child bearing C-cup I've seen. t*ts sag, faces wrinkle, jaws slacken, we age and die, deal with it.
http://www.007b.com/bra_sagging.php
"One study published on the subject of bras and sagging was done in 1991, in Japan. The study suggests that a
bra can actually increase breast sagging rather than the opposite. The abstract says (emphasis mine
"Eleven adult female subjects aged 22-39 years wore a certain brassiere for 3 months while anthropometry and moire fringe photographs on the anterior trunk were taken regularly once a week. After the 3 months, the brassiere was not worn for another 3 months. Then the measurements and photogrammetry were repeated for comparison using superimposed moire configurations. The results are summarized as follows. Regardless of slim or obese trunk, subjects with pendent breasts showed the highest degree of breast form "correction" from wearing the brassiere. In all subjects, after 3 months of brassiere constraint, the underbust circumference was smaller but the chest circumference became enlarged, the distance between the right and left nipples became wider, and the breasts tended to hang down. This change was more marked in obese subjects with pendent breasts. And when this type of subject wore a "well-fitted" brassiere for a long time, her breast form became developed, that is, her breasts hung down more."
Ashizawa K, Sugane A, Gunji T Institute of Human Living Sciences, Otsuma Women's University, Tokyo, Japan:
Breast Form Changes Resulting From A Certain Brassiere Journal of Hum. Ergol.(Tokyo) 1990 Jun; 19(1

53-62."