21st written as a word

the twenty-first century 二十一世纪

the day on which a person reaches the age of 21 and is traditionally said, in Western societies, to become an adult

It's Jamie's twenty-first next Friday. 下周五是杰米的21岁生日。

Her dad gave her a car for her twenty-first. 她21岁生日时她父亲送了一辆车给她。