For a milestone birthday, the design of the invitation generally focuses on what makes that particular birthday year memorable, according to Martha Stewart. For a 21st birthday, the invitations often highlight the theme of cocktails, since 21 is the legal drinking age in the United States. That theme can be geared towards the personal taste of the celebrant, showing festive champagne flutes, big mugs of frothy beer or fancy cocktails.
When designing a birthday invitation, there are basic elements that must be included, such as the event's date, time, location and the specific occasion. If you need a head count, include contact information for invitees to RSVP. If the occasion is a potluck, be clear about that on the invitation. Because the 21st birthday is commonly associated with alcohol, if the occasion will not feature alcohol, let guests know ahead of time.
Beyond the basic elements, birthday invitations are a wonderful place to have fun and express the celebrant's personality. There are many online resources available to help with invitation design. Greeting card stores or craft shops also offer design selections.
When designing the invitation, take the tone of the party into consideration. A more formal 21st birthday party might have spangled decorations with champagne. A more casual function might be simply decorated.