I think it depends in great part on how long you have been overweight. As Katydid said, older skin is just less elastic--it has a harder time returning to its former tightness and may never be able to.
That said, I have only a littlel saggy skin and I'm 49. I think this is mostly due to the fact that I was only a LITTLE overweight until my mid-30s. Then I was obese for a few years, I lost the weight once, gained it back over the course of 10 years, and then lost it again (the LAST time!). So while I did gain a significant amount of weight, it was only on for a few years each time.
I will say, though, that while pretty much ALL the saggy skin in my belly area tightened up (b/c, I guess, I never gained weight in that area until I was VERY fat), the saggy skin in my butt is still there.
That's an area I have been griping about since my teen years, so I guess I just had more stretching there.
I think that if it really bothers you, and you can afford it, there's not a THING wrong w/ having surgery to remove it. I can't spend $$ on stuff like that, so I won't be doing that. I make sure to "oil up" twice a day w/ a very good skin lotion, and I hope that over time maybe that will improve things somewhat. But overall I think I've been pretty fortunate.
All to say that YMMV! The most important thing to keep in mind, of course, is that you will still be getting 100% of the health benefits of losing weight even if you only get 89% of the aesthetic benefits.