Even during Prohibition, under the 18th Amendment, CONSUMPTION of alcohol was NOT ILLEGAL! Only the manufacture, transportation or sale of alcohol was banned.
Alcohol is a controlled substance controlled by the ATF, liquor licenses are required, proper ID is required and restrictions that are controlled by the STATES determine WHOM can purchase and consume the substance.
A person of legal age cannot be served an alcoholic beverage UNLESS and UNTIL he or she can produce evidence of their age in most states.
Again, using alcohol or being addicted to alcohol IS NOT UNLAWFUL!