Hi bobbosgirl. You're young so that will make the surgery easier, with fewer risks. I've known people to undergo the surgery at older ages and they've done well.
Weight gain from an underactive thyroid is tough to battle with diet alone.
To me, there is one major concern. If you don't address the underlying reasons for your weight gain (other than the thyroid problem), you will regain the weight and possibly risking damage to your stomach.
Lapband surgery forces the body to reduce food intake, but it does not go to the source of the problem which begins in the brain. Our beliefs and attitudes about food control our eating habits.
We eat for more than sustenance. Eating, because it is so pleasurable, becomes a way of self-medicating our minds and bodies. Please be sure to seek counseling before, during and after surgery. And consider getting a second opinion while talking to others about their experiences with lapband surgery. There are weight loss/health forums online that address lapband surgery specifically.