Here's a good dress up- party idea for any age: Select a letter of the alphabet (perhaps the first initial of the birthday boy or girl's name) and invite guests to come dressed as
something which begins with that letter! Easy!
This way you can still incorporate a theme however costumes will undoubtedly be varied! For example a friend of mine (17 years old) recently had a "P" party- here's who/what showed up: Power Ranger, Pocahontas, Pamela Anderson, Pebbles, Prehistoric Woman, Punk, Patrick Rafter, Photographer, Posh Spice, Princess, Patsy (from Ab Fab), Poison Ivy, Protestor, The Pink Ladies, Playboy Bunnies, Priest....etc You can go all out and also serve food and drinks or decorating in a color beginning with the letter you choose!