South Beach authorities and businesses will often "bend the rules", as long as people are well behaved and having a safe good time. However, one rule that is definitely not bent is underage drinking. Miami Beach police are very strict on this, and most of the local businesses are similarly strict on ensuring all customers are of age and have legitimate identification with them to prove it. Many businesses use a rule of thumb of "If you look under 30, you need ID".
And businesses are also strict on what forms of ID are acceptable. Acceptable IDs include U.S. state driver's licenses, and passports. Non-U.S. driver's licenses are often not accepted, as it's too difficult to be sure any such license is legitimate.
South Beach business doormen (who check ID prior to gaining admittance to a business) are usually very good at spotting fake IDs. Any attempt to use fake IDs will usually result in immediate confiscation, and perhaps more serious ramifications.
It's unfortunately common that a group of people will be walking around South Beach, and will be turned away from the fun at some establishment because one member of the group didn't bring their ID. And such groups may even be well into their 30's.
So, here's a good guideline...unless you look like you probably have grandchildren...bring your U.S. driver's license, or your official passport if you're from outside the U.S.
