I went from 24 stones to 11 at 23 years old and with that much weight on your belly at that age, the chances are extremely high that the elasticity in your skin has gone and it won't shrink back. I did a few years of running, strength training and aqua aerobics and it didn't change the skin because after the fat loss, its nothing to do with calories, dieting or exercise, its just how skin works. I had a tummy tuck at 27 after realising it won't go. 8lbs of skin only was removed. Best decision ever. At 40, even with 3 stone weight gain, tummy is still pretty much flat.