Hyphenate compound cardinal and ordinal numerals from twenty-one (twenty-first) to ninety-nine (ninety-ninth) when they are written out: There are twenty-nine members on the committee.

Do you write 21st century or twenty-first century?

twenty-first century? My short answer for all specified contexts is twenty-first century. Unless the name of the century begins a sentence or is part of a proper name, it is written in all lowercase letters: We are living in the twenty-first century.

Should 20 year old be hyphenated?

When to Hyphenate Year Old

That is, when the phrase is describing the age of a person, place, or thing, and it precedes that noun in a sentence, then it should be written as year-old. In such cases, a hyphen should also connect year old to the number that precedes it (for example, “20-year-old girl”).