9岁的孩子英语怎么说

9岁的孩子英语可以用"a nine-year-old child"或者"a child who is nine years old"来表示。这个表达方法源自英语中常用的表示年龄的方法，即在数字前加上"year-old"的形式来表示。
例句：
My sister is a nine-year-old child. (我的姐姐是一个9岁的孩子。)
The boy who is playing soccer is a nine-year-old child. (正在踢足球的男孩是一个9岁的孩子。)
I have a nine-year-old daughter. (我有一个9岁的女儿。)
The nine-year-old child is very smart. (这个9岁的孩子非常聪明。)