Definition

1. Adjective:
   • Being four years of age: Describes a person, animal, or entity that has lived for four years.

Usage

• The word "four-year-old" is primarily used as an adjective to specify the age of a noun. It is typically hyphenated when used before a noun. When used as a noun phrase to refer to a person or animal of that age, it is often written with hyphens (e.g., "a four-year-old").

Examples

• Adjective:
  • She has a four-year-old daughter. (The adjective "four-year-old" describes the noun "daughter".)
  • The four-year-old car still runs perfectly. (Here, it metaphorically describes the age of the car.)
  • He is four years old. (Note: In this predicative position after the verb "is", the phrase is not hyphenated.)

Advanced Usage

• As a nominalized adjective (noun phrase): The hyphenated form can function as a noun to refer to a person or animal of that age.
  • The four-year-old was very excited for her birthday. (Here, "four-year-old" acts as a noun referring to the child.)
• Plural form: When used as a noun, the plural is formed by adding "s" to "year" (e.g., "four-year-olds").
  • The class is for four-year-olds.

Variants and Related Words

• Four years old: The unhyphenated phrase used after a linking verb like "is," "are," "was," "were."
  • My son is four years old.
• Four-year-olds: The plural noun form.
  • The playground is designed for four-year-olds.

Synonyms

• Aged four: (Formal) Having reached the age of four.
• Four years of age: (Slightly more formal) Having lived for four years.

Notes

• The hyphenation rule is standard for compound adjectives preceding a noun (e.g., a four-year-old child). Omitting the hyphens in this position ("a four year old child") is generally considered incorrect in formal writing.
• The phrase can describe non-living things to indicate their duration or existence (e.g., a four-year-old company, a four-year-old policy).

Adjective

1. four years of age