Depending upon the likes of the teen, you can decorate in a sports theme, a dancing theme, a musical theme, a cheerleading theme, a movie theme, a "best friends" theme, or a garden theme. Use whatever the teen likes.
You can use many of the same themes for teens as for children; they just need to be more sophisticated. A safari party would use animal print chair covers, serve zebra cake, and guests provide gifts for the "party animal." Or, go with a nightclub theme complete with disco ball, soda in bottles, rock band posters and a dance floor.
Well it all depends on whether the birthday child would like a themed party or not. Some neutral birthday decorations can be helium balloons with HAPPY BIRTHDAY written on them or even birthday banners. The same decorations can be used for an adult.