Aren't you always allowed to refuse to be breathalyzed? This even applies if you're operating a vehicle recklessly and the officer pulls you over while you have a 40 in your hand. (In that situation, the officer can still arrest you, but you'll be given the more accurate blood test at the station.)

However, if you're just at party, I think the testing is simply out of question. Also, unless your state has stop and identify laws, you wouldn't have to identify yourself and show that you're 21 or older either.

You can refuse to be breathalyzed. But, at least in my state, that is admissible as evidence of guilt of DUI (but if you agree to be breathalyzed and you blow within twice the legal range, under .16, the state will drop charges after you complete the program of community service, classes etc. They won't drop charges after completion of the program if you refuse to be breathalyzed). And, you can only refuse one time. The next time you get pulled over, it's a separate misdemeanor to refuse again. It's pretty much always in your best interest to be breathalyzed.

You technically can refuse a breathalyzer or chemical test, but check your state laws for repercussions. Some states have some form of Implied Consent—i.e., if you are agreeing to hold a license and drive on the roads, it is implied that you will consent to some form of BAC test. I'm not an attorney, but from what I understand it can be just as bad as blowing over a .08 in some states.