It might be a state by state, country by country thing. I know in Virginia it is the day before your 21at birthday based off some court case explaining the 365 days = a year thing.
When does a person reach their 21st birthday?
The attorney general’s office has opined that a person attains his/her next year of age on the day prior to his/her birthday. (1963 Va. AG LEXIS 215; 1962-1963 Op. Atty Gen. Va. 87)
It might be a state by state, country by country thing. I know in Virginia it is the day before your 21at birthday based off some court case explaining the 365 days = a year thing.
https://www.abc.virginia.gov/licenses/retail-resources/preventing-underage-sales