If what you say is true, then you're basically saying I have no hope!!
I'm 25, but I smoke and I've been overweight all my life. I naturally always had a small stomach (in relation to the rest of my body), but after I had my child I was completely stretched out, and have had to live with stretch marks on my stomach and hips since. And I naturally have extremely big thighs, so that would be a cause for concern in relation to what you are saying.
But I will say that, I've lost 20lbs so far, and in watching my stomach getting smaller, I can see my stretch marks coming closer together, and my lower abdomen area doesn't hang as much. So I can only assume that my skin is tightening back. I promise you it's not muscle; it still feels like putty.
I can understand that if your skin is ridiculously stretched (like someone having sextuplets, for example) that there's no way your skin can come back from that; but my legs have been massive for almost all of my life, (to the point where even if I were to reach my goal weight, I will not have stick thin legs), and I have no stretch marks on my legs. And I think the skin will tighten back, along with the muscle filling it out, as you said.
I've seen some heavy-set men who have great skin; no stretch marks or anything. So I think it's relative to the type of skin you have as well, even in some cases, your ethnicity can determine your skin-type and how well it can heal. Of course age would make a difference (As in the example of starr jones) but even in your 30's, I dont think it should make a huge difference in healing capabilities; maybe a little more difficult, but still very possible.