The correct expression is '9-year-old son' or 'nine-year-old son'. Note the hyphens. Why? Here's the rule: When you make a compound adjective or a compound noun involving a number and a measure word ( a unit of time, money, or distance, for example), the measure word is in the SINGULAR. Here's a little exercise to practise this rule. What do you call.. 1. A son who is nine years old ---> a nine-year-old son 2. A bill which has a value of five dollars? 3. A run which is five miles long? 4. A tent which sleeps five people? I'll look forward to getting your answers.

고마워요! 완벽히 이해했습니다.

형용사로 명사에 대한 추가적 설명을 할 때 "아홉살 된"..이라고 할 땐 a 9-year-old son. 하지만 2형식 처럼 be 동사 뒤에 형용사 자체로만 사용할 때는 Today, he turns 9 years old. 화이팅!