Why is turning 21 such a big deal in usa (please read description before judging me)?

Anonymous
i am from Deutschland (Germany) and moved to california at middle of June with my family. i remember a classmate from school saying they are celebrating her sisters 21 birthday and another student saying they were celebrating the same thing except her brothers 21 birthday a week some after. why is turning 21 a big deal in usa? please do not judge me because i am new to this country from living in a different country and still getting used to the ways and differences from on the other side of the world for 18 years and we never celebrate a persons 21 birthday like americans do

sorry about my English. i am not very good at it because i don't speak English as a first language
Why is turning 21 such a big deal in usa (please read description before judging me)?
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