The food for a 21st birthday will depend on what type of place you are having the party at.

If your having it at a bar or club, you will want to order food ahead of time to be set up in a buffet style near the area of your celebration. If the establishment doesn’t offer food, ask them if you can bring your own or have it catered.

If you are having it at home, you can stock every area where people might congregate with bowls of finger foods to give them a snack at every turn. Use different items like nuts, pretzels, and chips with a few bowls of fruit around for those who are health conscious.
Have a main dish like BBQ sandwiches on buns in a crock pot so you don’t have to worry about it getting cold or not being warm enough to stay safe at a party. Then all you have to do is supply paper plates and a bag of chips for the main course. This should be enough to keep all your guests happy. If they don’t eat right away, don’t despair, once the games involving liquor start, people will find themselves needing a bite to eat so keep it out and hot for the whole party.