Used as a noun, your twenty-first is your twenty-first birthday, or the party you have for it. This is very British and Australian.

Used as a noun當名詞用時，your twenty-first你的twenty-first就是 your twenty-first birthday你的二十一歲生日，or the party you have for it或你為它辦的派對。This這 is very British and Australian是很英國、很澳洲的說法

Historically, turning 21 was the age you became a legal adult in Britain, so the party was a big one — bigger than 18 — and even though the legal age moved to 18 long ago, the 'twenty-first' kept its special feeling.

Historically在過去，in Britain在英國 turning 21滿二十一歲 was the age you became a legal adult是成為法定成年人的年紀，so所以 the party這場派對 was a big one辦得很大——than 18比十八歲的 bigger更大——and even though而即使 the legal age法定年齡 long ago很久以前 moved to 18就改成了十八歲，the 'twenty-first' kept its special feeling仍保住了它特別的感覺

People say 'It's my twenty-first next week' or 'I'm going to Ben's twenty-first'. Written down you will see both 'twenty-first' and '21st'.

People say人們會說 'It's my twenty-first next week'下週是我的二十一歲生日 or 'I'm going to Ben's twenty-first'我要去參加Ben的二十一歲生日派對。Written down寫下來時 you will see both兩種你都會看到：'twenty-first' and和 '21st'

Of course the word also just means 'number 21 in an order' — the twenty-first floor, the twenty-first century — but as a thing you can have and go to, it means the birthday.

Of course當然 the word這個字 also just means也單純表示 'number 21 in an order'「順序裡的第二十一」——the twenty-first floor二十一樓、the twenty-first century二十一世紀——but但 as a thing you can have and go to當它是一件你可以「有」、可以「去」的東西時，it means它指的就是 the birthday那場生日