Don't worry, we've visited twice (once when I was 9 and then just two months ago) and both times I've had a great time!
As others have said, the shopping is good, the food is good and the shows are good. Make sure you go to the Beatles Love and Viva Elvis if you can. If you don't like those shows, there are plenty of other Cirque du Soleil shows to see!
Each casino has its own little quirks and interesting things to see, such as the lion habitat at the MGM, dolphins at the Mirage and just walking through the casinos themselves or walking around outside is fun.
They have the fashion mall just opposite the Wynn which is pretty good
Circus Circus has lots of free circus acts as well as the Adventuredome indoor theme park.
You could go skydiving if you want or do a helicopter tour or grand canyon tour or something like that.
But honestly don't think that you're missing out on anything by being a teen. Seriously the gambling is not that interesting anyway IMO and the best parts of Vegas are the entertainment, the majority of which is open for all ages.