Adult birthday parties don't share one default format the way a kids' party mostly does — a casual backyard hangout, a dinner reservation, and a night out all ask something different of guests, and the invitation should make that obvious immediately. The wording is less about age and more about what kind of evening a guest should actually show up expecting.
A backyard get-together or a low-key house party benefits from wording that matches the relaxed vibe — this is where personality can lead, as long as the time and address are still unmistakably clear. Guests mainly need to know if it's come-and-go or a set start time, and whether to bring anything.
Anything reservation-based needs a firmer RSVP than a casual hangout, since a restaurant headcount usually has to be finalized in advance and can't flex the way a backyard party can. Say plainly that it's a reservation and that a deadline is firm, not a soft suggestion.
A bar crawl or a night out involving multiple stops needs practical detail a dinner or backyard party doesn't — a clear meeting point and time, and often a rough plan for what happens after the first stop, since not everyone will want to stay for the whole night.
A party at someone's home sits between the fully casual hangout and a formal dinner — guests need to know if it's a drop-in or a seated occasion, whether kids are included, and anything about parking or noise (useful for neighbors, and for guests deciding how late to stay).
The tone can shift entirely between these four, but the underlying essentials don't — date, time, full address, and a clear RSVP deadline still need to land before any of the personality does. See our full guide to writing a birthday party invitation for those basics, or our guide to milestone birthday invitations if the age itself is the occasion.