The laws that govern legal age to buy and consume alcohol vary depending the state and the are modified with the passing of time. In many states, there are different ages for categories of alcohol. For example, today you would be able to buy beer at 18 but wine and spirit drinks only after the subject turns 21 in most states. But, in the seventies, most states forbid any kind of consumption and sale and over the years some adjustments were made
Since the repeal in prohibition that forbid production, importation and consumption of alcohol from 1920 to 1930, there were plenty of modifications but there are still some states in which legal age to consumption and purchase of alcohol is still of 21. California drinking age is still of 21.
People under 21 cannot be in locations where alcohol drinks are being sold and cannot possess alcohol in public places as their hosts or bar operators amay be held liable for the disturbing behavior of an underaged that got drank because of them. Jerry Brown, governor of California, lower the law to taste alcohol to 18 years old, as long as they are part of a brewery of making wine course and they have to spit it right after.