I think it comes down to luck - how much collagen you have in your skin and how much weight you have to lose. It reduces with age - so in that respect, at 21, you are far far more likely to have plenty of it, and thus have less excess skin post weight loss.
At 25 or so, I lost about 40 kilo, and had absolutely no excess skin. At 45, I've lost 70 kilos, and have enough that I don't like it, but not enough to do anything about it - its mostly hidden by long shorts and a shirt.
I don't think there is a damn thing you can do about it, really - you either get it or you don't. I do think that exercise and muscle tone helps drape things nicely, but it doesn't affect the skin itself....it's certainly helped shape things in my case, but it doesn't relieve the problem, and there is absolutely NO evidence that diet or lotions and potions help.
There are plenty of surgical options if it is a problem later - but at your age, you are in with a far better chance of minimal problems than us oldies!
