With her parents in the middle of a divorce, underage Disney pop princess Miley Cyrus, 17, threw back a Corona cerveza at a club in Madrid, Spain last night ... where the legal drinking age is technically 18 -- but there's a catch.


TMZ contacted several Spanish authorities who each confirmed the law ... and then made it clear that underage drinking is simply no big deal in their country.

One police officer told us ... people as young as 16 are cool to consume "certain" beverages in Madrid -- but the cop refused to explain what those beverages are. 

In that case: Salud!