“The legal age for alcohol in the USA is 21 years old. The National Minimum Drinking Age Act of 1984 required all states to raise their minimum purchase and public possession of alcohol age to 21. States that did not comply faced a reduction in highway funds under the Federal Highway Aid Act. The U.S. Department of Transportation has determined that all states are in compliance with this act. The national law specifically prohibits purchase and public possession of alcoholic beverages. It does not prohibit persons under 21 (also called youth or minors) from drinking.”
State laws vary.
My understanding is that in Oklahoma we do not have a specific law banning underage drinking, however, we do have several other laws against possession and purchase and having a measurable BAC that effectively outlaws it.