It's not a federal law, you can buy and drink alcohol at 18 in the Virgin Islands and Puerto Rico for instance, so definitely possible to drinking at 18 legally in the USA. I don't know if there is a federal drinking age but it's definitely not above 18.

Also in I want to say about half the states (could be wrong here, but at least a few), it is legal to drink well below 18 in a private home.

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Example, wisconsin:

>Can an underage person possess and consume alcohol beverages on licensed premises? Yes. Persons under age 21 may possess and consume alcohol beverages if they are with their parents, guardians or spouses of legal drinking age; but this is at the discretion of the licensee. The licensed premises may choose to prohibit consumption and possession of alcohol beverages by underage persons. (Sec. 125.07(1), Wis. Stats.)

The drinking laws in at least ~half the USA are a lot looser than most people think. If the parents are ok with it the kid can generally drink somehow.

https://www.revenue.wi.gov/Pages/FAQS/ise-atundrg.aspx#undrg...

It’s essentially a federal law - if a state wants to get full federal funding for highways, they have to have a law restricting alcohol purchase and public consumption to 21+. It’s from the National Minimum Drinking Age Act of 1984.