Writing Age: 5-year-old vs. 5 Years Old
Hyphenate an age when the age phrase works as a compound modifier before a noun or as a noun itself: “a 5-year-old child” or “a 5-year-old.” Leave the phrase open after a linking verb: “the child is 5 years old.”
Write “a 5-year-old child” and “a 5-year-old,” but “the child is 5 years old.” The grammatical role determines the hyphens.
Key details
Important caveats
Predicative Boundary
Do not hyphenate the ordinary “years old” phrase after the noun or linking verb: “She is 5 years old.”
Further guidance
Modifier Rule
Use hyphens to bind the age phrase when it modifies a following noun or stands as a noun phrase.
Practical Application
Use a hyphenated age expression when it modifies a following noun or functions as the age-noun phrase, but leave “years old” open when it follows a linking verb.
Reader Check
Ask what grammatical job the age phrase is doing: before a noun, standing as the noun phrase, or following “is/was” as a predicate?
Worked Example
Write “a 5-year-old child” and “the 5-year-old,” but write “the child is 5 years old.”
Sources and evidence
Sources are shown with the role they play in this guide. Historical or style-sensitive claims are kept within the evidence boundary described above.
- Hyphens, En Dashes, Em Dashes — age terms Q&A (opens in a new tab)The Chicago Manual of Style · Chicago guidance on hyphenating age expressions when attributive or nominal, versus open predicative age phrases