When numbers are used as the first part of a compound adjective, use a hyphen to connect them to the noun that follows them. This way, the reader knows that both words function like a unit to modify another noun. This applies whether the number is written in words or in digits.
However, a hyphen is not required if the number is the second word in the compound adjective.
Hyphen in Compound Adjective With Fractions
When using a fraction (e.g. half or quarter) as part of a compound adjective, it should be hyphenated so the reader understands which fraction is modifying which noun.
Hyphen With Number of Years
Use hyphens when you are using someone’s age as a label.
Don’t use hyphens when you’re just talking about a span of time.