Before a noun, use hyphens and the singular: a five-year-old girl. After the noun, use no hyphens and the plural: the girl is five years old. The hyphenated form is always “year-old”, never “years-old”. This guide covers the rules, the noun form (a five-year-old), common mistakes, and the number style used by different writing guides.
Written by Mitch Rankin, CEO and owner of EnglishForward.com since 2018. Last reviewed 9 October 2026.
The rules
1. Before a noun: hyphens, singular “year”.
The whole phrase works as one adjective in front of a noun, so the words are joined and “year” stays singular, like “a ten-minute walk”.
- She has a five-year-old daughter.
- They bought a 100-year-old house.
- It is a ten-year-old company.
2. After the noun: no hyphens, plural “years”.
When the age comes after “is”, “was” or another verb, it is not squeezed in front of a noun, so there are no hyphens and “years” is plural (for any number above one).
- Her daughter is five years old.
- The house is 100 years old.
- Our company is ten years old today.
3. As a noun: hyphens, and add -s for more than one.
When the phrase stands for a person, the noun is understood and the hyphens stay.
- The five-year-old loved the playground.
- The programme is for ten-year-olds.
Quick reference
| Position | Form | Example |
|---|---|---|
| Before a noun | n-year-old (singular) | a 6-year-old child |
| After the noun | n years old (plural, no hyphens) | The child is 6 years old. |
| Standing for a person | n-year-old | The 6-year-old ran outside. |
| Several people | n-year-olds | Activities for 6-year-olds |
| Age of exactly one | one year old | The baby is one year old. |
Common mistakes
1. “Years” inside the hyphenated phrase
- Not correct: a ten-years-old boy
- Correct: a ten-year-old boy
- Also correct: The boy is ten years old.
2. No hyphens before the noun
- Not correct: We bought a thirty year old car.
- Correct: We bought a thirty-year-old car.
Some publications, especially in British English, leave out the hyphens in informal writing. The hyphens are the safer, standard choice, and they make the meaning clearer.
3. Hyphens after the noun
- Not correct: The boy is ten-years-old.
- Correct: The boy is ten years old.
Figures or words?
Style guides differ, so follow the one your school, newspaper or employer uses.
| Style | Rule for ages | Example |
|---|---|---|
| AP style (US news) | Always figures | a 5-year-old girl |
| Chicago style (books, academic) | Usually words for numbers under 100 | a five-year-old girl |
Whichever you choose, be consistent in the same document.
The same rule for other units
The pattern works for any measurement used as an adjective before a noun: the unit stays singular.
- a two-week holiday (not a two weeks holiday)
- a three-hour flight
- a six-month-old baby, but the baby is six months old
- a 10-kilometre run
Frequently asked questions
Is it “year old” or “years old”? Use “year” (singular, with hyphens) before a noun, and “years” (plural, no hyphens) after it.
Why is “year” singular in “five-year-old”? In a compound adjective the unit stays singular, as in a five-minute break.
How do I write ages in a list or form? Write the number only (“Age: 6”) or “6 years old”. Hyphens are not needed.
What about a baby under one? A six-month-old baby (before a noun), the baby is six months old (after).
One common mistake English learners make is forgetting the hyphen when an age is used before a noun. For example, write “a 10-year-old boy,” but “the boy is 10 years old” because the age follows the verb and is not acting as an adjective. A simple rule to remember is that age descriptions before a noun use a hyphen and “year” stays singular.
Related guides
Have a hyphen question? Ask it in the English Forward community thread
Clap