Answer:
Do a "When You Were Fifteen" party. The party is done based on what was popular 15 years ago when the person having the birthday was still 15 years old. Find a high school yearbook and use the photos in the invitations and decorations. Play the top songs of the year and encourage people to dress similar to the way they dressed when they were fifteen. This is great for a multigenerational party, since all of the costume styles will be as if each guest was still fifteen. To keep in the spirit of a teen party, serve finger foods, and let the guests mingle with the rule that they have to find out three new things about the birthday boy or girl to share at the end of the night.