After thirty, the body begins to show subtle changes. My own experience is that the back of the arms is first to "rebel," followed by the waist and abdomen. Some people see this as "getting old," but I prefer to understand it as your body telling you: it's time to adjust your strategy. At this stage when choosing formal dresses, the cut matters more than anything else.
For those over fifty, I don't have first-hand experience, but my mother taught me a lesson. She said: "At this age, dressing well means first and foremost not shortchanging yourself." She wore a burgundy silk dress to a class reunion—not flashy at all, but she looked absolutely radiant. Comfort and presence might truly matter more than any cutting technique.