Is there a hyphen in “year old”?

Phrases describing age, such as “one-year-old,” are hyphenated in some contexts but not others.

Hyphenate when the phrase functions as an adjective before a noun (e.g., “two-year-old daughter,” “six-week-old puppy”) or when it functions as a noun (e.g., “he behaves like a four-year-old,” “the 10-year-old was staring”).

Don’t hyphenate when the phrase is used as an adjective after the noun. In this case, the unit of time is also pluralized (e.g., “years”) unless the number is one. For example, “my daughter is two years old,” “he behaves like he’s four years old,” “the puppy is one month old.”