As a plastic surgeon who offers Thermage, I have a unique perspective in that I can also offer surgical procedures when those are the better choice. Results do vary with Thermage so no one can promise you what your own results might be, and especially without seeing you personally. As we get more experience with Thermage, we are better able to predict who is going to get a better response to the treatment.
Generally, it is people with some elasticity in the skin, which diminishes with age so younger is better. If you have stretch marks, or have had a lot of weight loss, it is much less likely that you will be happy with it. Bottom line is that your doctor who will be doing it is in the best position to advise you about results, since he or she has actually evaluated your skin.