Hi guys! Also here to jump on this train I'm not a doctor by any means, but general consensus from posts/journals I've skimmed suggests that the jaw/cheekbones finish most of their growth by 18-22. (At least in females -- sometimes more like 25 for males?) I think best bet would be to ask a bunch of surgeons, and compare all of their answers (hopefully not all lying!)
Regarding sagging, it seems like skin elasticity is important but maybe less important than how skillfully the surgeon re-drapes the skin? If anyone could weigh in on this I'd be grateful!