How likely am I to have loose-skin from weightloss?

I suppose, foremost, my question is: what constitutes massive weight-loss?

If I anticipate losing 40–60 lbs over the span of 2 years, through a healthier diet and more exercise (including weight-lifting), how likely is it that I’ll have saggy skin? I’m still quite young, but I do have stretchmarks and I know that that has to affect quite a bit.