The question of "year-old vs year old" trips up writers in contracts, press releases, HR documents, and everyday emails. The answer isn't arbitrary. English has a clear rule for compound modifiers, and once you see how it applies here, you'll get it right every time without having to think twice.
The core rule: what a compound modifier is
A compound modifier is two or more words that work together to describe a single noun. When that combination comes directly before the noun it modifies, you hyphenate it. When it comes after the noun (or after the verb "to be"), you don't.
That's the whole rule. "Year-old" follows it exactly.
- Before the noun (hyphenate): "a five-year-old contract"
- After the noun or verb (no hyphen): "the contract is five years old"
Notice that when the phrase moves after the verb, "year" also becomes "years." The two changes travel together. "The employee is 30 years old" is correct. "The 30-year-old employee" is correct. "The 30-year old employee" (one hyphen missing) is not.
Why the hyphen matters in professional writing
Without the hyphen, the reader briefly has to parse whether "year" modifies "old" or the noun directly. In fast-read contexts like executive memos or legal documents, that half-second of ambiguity costs you. Consider: "a 10 year old policy." Is it a 10-year-old policy (one that's been in place for a decade) or something else? The hyphen removes the guesswork.
Good style in titles and headings depends on the same logic: modifiers that run together before a noun need to signal their unity to the reader. The hyphen does that work.
Common patterns you'll actually encounter
Here are the forms that show up most in business writing, with the correct version in each case.
Job postings and HR documents. "We're looking for a candidate with 10 years of experience." No hyphen, because "years" follows the noun phrase. But: "This is a 10-year-old vacancy that we've never properly filled." Hyphen, because the compound modifier precedes "vacancy."
Legal contracts. "The 25-year-old agreement will be reviewed in 2027." Hyphenate. "The agreement is 25 years old." No hyphen.
Press releases. "The company's flagship product, now 15 years old, leads the market." No hyphen after the verb. "The 15-year-old flagship product leads the market." Hyphen before the noun.
Referring to a person's age. "She is 42 years old." No hyphen. "The 42-year-old CFO joined the board last quarter." Hyphen. This one is where writers most often slip, partly because "42 year old" sounds right when spoken aloud.
The "years old" trap
When the phrase moves to a predicate position, "year" becomes "years" (plural) and the hyphen drops. This is a double change, and writers often make only one of the two corrections.
Wrong: "The building is 100-years-old."
Wrong: "The building is 100 year old."
Right: "The building is 100 years old."
The hyphens in "100-year-old" exist because the phrase is doing modifier work before a noun. Once you shift it to a predicate ("is 100 years old"), it's a plain adjective phrase, not a compound modifier, so both the hyphens and the singular "year" go.
What about "two-and-a-half-year-old"?
Extended compound modifiers follow the same rule. "A two-and-a-half-year-old project" gets hyphens throughout, because the entire stretch ("two-and-a-half-year-old") is doing the work of a single adjective before the noun "project." Move it to predicate position and you write: "The project is two and a half years old." No hyphens, plural "years."
This matters in business writing when describing things like funding periods, lease terms, or warranty windows. "A three-and-a-half-year-old lease" vs "the lease is three and a half years old." The structure holds regardless of how long the modifier gets.
A quick test you can run in any sentence
Ask yourself two questions. First: does the age phrase appear directly before a noun? If yes, hyphenate and use the singular "year." Second: does the age phrase follow the noun or a form of "to be"? If yes, drop the hyphens and use "years" (plural).
This two-question test covers the vast majority of cases you'll face. It's the same logic that governs phrases like "well-known" and "well known," where position before or after a noun determines whether the hyphen appears. If you want to see that pattern in detail, the article on "a well-known problem" works through exactly the same compound-modifier logic.
Ordinal ages and headlines
One edge case worth knowing: when "year-old" becomes a standalone noun (as in "a group of five-year-olds"), you hyphenate and drop the final noun. The phrase itself becomes the noun. "The five-year-olds in the study" is correct. "The five year olds" is not.
In headlines, the same rule applies. "30-Year-Old CEO Steps Down" uses title case and hyphens because the compound precedes the noun "CEO." In sentence case, the same headline reads: "30-year-old CEO steps down." The hyphen stays; only the capitalisation changes.
The rule doesn't shift based on the size of the number, the formality of the document, or the style guide you're working under. The Chicago Manual of Style and most major editorial guides align on this one. Compound modifiers before a noun get hyphens. After the noun or verb, they don't.
Get those two positions clear in your head, and "year-old vs year old" stops being a question you need to look up.