The momentum of the 18th and 21st birthdays carries the parties without the need for any kind of formal theme. However, on the 20th birthday, as the celebrant reaches his second decade, it's fun to use a theme with special connections to the number 20 for a memorable birthday party.
Roaring 20s Party
Invitations for a roaring 20s party in honour of a 20th birthday should contain a secret password admitting the guest to the "speakeasy." Of course, all of the drinks will be imitation cocktails; choose from a selection of these from such sites as Cocktails Non Alcoholic. Create a playlist from the following artists suggested by Celebrations: Paul Whiteman, Duke Ellington, Louis Armstrong and others; another suggestion is to provide a dance instructor who will teach everyone the Charleston and the foxtrot. For 104 minutes of campy entertainment you could show the Raoul Walsh film, "The Roaring Twenties," starring Jimmy Cagney.
20th Century Heroes and Rogues Party
Anyone who is celebrating a 20th birthday will have lived at least half his life in the 21st century, so time-travelling back to the 20th century and channelling well-known individuals from that time can make for a spirited theme party. You could assign each guest or group of guests a particular decade and ask them to dress as either a hero or rogue from that particular decade. Provide entertainment by playing charades based on 20th century events involving famous or infamous people; for example: Amelia Earhart's disappearance, Richard Nixon's resignation or Neil Armstrong's walk on the moon.
Japanese Coming-of-Age Party
The age of majority in Japan is 20; a holiday cailed Seijin no hi celebrates this benchmark in young people's lives with coming of age festivals celebrated in January all over the country. A 20th birthday party with a Japanese coming-of-age theme could feature a variety of Japanese food, including sushi. Some of the food could be prepared ahead of time or catered. Guests could cook other easy-to-prepare Japanese foods as part of the party entertainment. One of the guests could practice folding paper cranes ahead of time and then teach guests how to make them, in keeping with the traditional Japanese myth that folding a thousand cranes brings good fortune. Guests could also choose a Japanese name just for the party, selecting from a list such as the one at Behind the Name.
A 20th birthday party based on the traditional 20-questions game offers a variety of possibilities. Guests might each prepare a 20-question trivia quiz on subjects such as sports, music or movies; teams of guests could exchange lists and work together to answer questions. Just for fun, the guest of honour could write 20 questions that she wants each guest to answer truthfully; each guest would get three passes for questions she didn't want to answer. Or guests could choose from 20 subjective questions; each guest would then write the question and her answer on an index card. The cards all go into a box; as cards are pulled out and read one at a time, the group tries to guess who wrote each. Sample subjective questions: "What is the best album of the last two years and why? Which would be more satisfying --- fame or wealth. Give your reasons."