'九岁' (jiǔ suì) literally combines the numeral 九 (jiǔ, 'nine') and the measure word for age 岁 (suì, 'years old'). It functions as a noun meaning 'nine years old' and is used exclusively to state a person’s age—never for objects, animals, or abstract concepts. Unlike English, Chinese does not use 'years old' as a separate phrase; 岁 itself carries that meaning, so 九岁 is a complete, self-contained age expression.

This term appears in everyday contexts like introductions, school forms, medical records, or casual conversation about children. It follows standard age-naming conventions: number + 岁, with no article, verb, or modifier needed. It’s neutral in register—appropriate for formal documents and informal speech alike—and always refers to completed years of life (e.g., a child turns 九岁 on their ninth birthday, not after nine full years have passed).