Welll, it depends on what they like. Say they like...Japan, then get them some thing from the Japan pavillion in WS. Or say they're into space. Get them something in like, a gift shop by mission space. It all depends on what they like to do. You can find pretty much anything at WDW. I hoped that helps!!!!
I think any type of jewelry would be a nice souvenir. Something that fits their personality. Definitely check out Tren-D in Downtown Disney, I would buy that store if I could. There are so many nice things in there that any teen girl would love.
It really depends on the person. I mean, once you go there and look around enough, you will find something. For my friends, I've gotten: good luck charms, jewellery, a shotglass (as a joke), stuffed animals and pins. It really depends.