In my late twenties I dropped 90 lbs over roughly 18 months, and skin trouble never became a big deal. The belly is where extra skin shows most (reminds people of a postpartum tummy, and I have never had a baby). A perfectly flat stomach is likely out of reach, and honestly Im fine with that. Two things I would suggest: more collagen in meals (jello counts) and lifting weights, since muscle gives the skin support.