four years old

four years old

8-year-old 是前置定语，放在名词前，如：
He is a 8-year-old boy.
8 years old 是后置定语，放在名词之后，如：
The boy is 8 years old.
希望对你有帮助！

如果中间有连字符-，你就不要在year后面加s ,也就是说，你可以说“he is four years old”或者“he is a four-year-old boy”

He is four years old。

The boy is four years old.

a four-year-old boy. he is four years old