Grammar Clinic: The REAL difference between YEAR-OLD and YEARS OLD

If you use a noun as an adjective, it must be singular

• You can say, “She is ten years old.” But if you use that phrase as an adjective, it becomes, “She is a ten-year-old girl.”
• The ceiling is ten feet high. The room has a ten-foot ceiling.
• My 16-year-old sister is so brilliant.
• A 45-year-old building

When speaking about age in English, we use the verb be (am, is, are) and not have/has.

• I’m thirty years old.
• My nephew is fourteen years old.
• These houses are 200 years old.

We can also say am / are / is + __(age)__ without “years old”:

• I’m thirty.
• My nephew is fourteen.

When it is somebody’s birthday, we say they turn __(age)__

• We threw a big party when my mother turned fifty.
• My youngest cousin just turned three.