Age and genes...that is a lot of it. Now in my late thirties, I am depending on genes.

I have had three BIG babies, and I have no stretch marks and my tummy seems to be taming -- not perfect, but pretty good. My OB says it means I have a lot of collegeen (sp?), and so I will age gracefully because I have a lot of natural elasticity in my skin (We will see...my breasts don't seem so elastic after close to eight years straight of pregnancy and nursing...). I have known women who were really big who ended up with no sag at all, but one of the legal assistants where I used to work only lost 20 lbs. and she definitely had some sag going on...but even hers got better after a few months as she continued to work out.
What I do know is that the younger you get the weight off, the better the skin issue is. That is even the case between the 40s and the 50s or the 50s and the 60s...and healthwise it is all good.