But the 21st birthday changes a person's legal status. Depending on your country and state, you may be able to [drink, drive, join the army, vote, etc.] two days after that birthday (your "age" is 21), when you could not one week earlier (your "age" was 20).
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Your 21st birthday is more than just adding another candle on the cake; it’s a cultural rite of passage — especially if you’re in college. Whether you’re stepping foot into a bar legally for the first ...
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Wikipedia and WHO, among others, seem quite happy to write "the 21st century" (with small c), and this the form I prefer. But it's your choice and no-one can criticise you for writing "the twenty-first century".
The suffixes -st (e.g. 21st), -nd (e.g. 22nd), -rd (e.g. 23rd), and -th (e.g. 24th) are used. In the Victorian period, these indicators were superscripts (2nd, 34th) under general French influence especially in British English, but by the late 20th century, formatting them on the line was favored. [citation needed] Since the 1990s, the superscript style has been revived somewhat because some ...