It is strictly prohibited for anyone under 21 to consume or purchase alcohol in the US. It is also forbidden to consume them on the beaches, and that goes for people of any age. Driving under the influence of alcohol is also strictly prohibited (as in Brazil), and the Miami police usually set up roadblocks and traffic blitzes in the city's roads and bridges, to catch those who do not comply with the imposition. The law there is pretty strict there, so don't risk breaking any of these rules, because you can get caught and the consequences are enough to spoil the whole trip.