Unfortunately there is no way to know ahead of time whether you'll have this problem. Being younger helps because your skin naturally has more collagen and elasticity than later in life, but even youth doesn't make everyone immune.
Once the damage is done it is all but impossible to undo it without surgery, and then you're just trading sag for scars
Your best bet is to start now and try to prevent it. Get yourself a good moisturizer, something with collagen added. The stuff joyfulloser suggested has been used forever by women to prevent sag and stretchmarks from the fast swell/deflate of preggie bellies and people swear by it, so it couldn't hurt to try them either
