没有 fifteen year old 与,fifteen-years-old,这两种用法 ,即它们的书写是错误的.fifteen-year - old 15岁的（ 相当于形容词,后要接名词）fifteen years old 只能作表语.如The boy is fifteen years old .= He is a ...fifteen years old是15岁，例如I am fifteen years old，而fifteen-year-old是十五岁大，例如my brother fiftten-year-old，而fifteen-years-old是指十五岁大的，例如fifteen-years-old should be allowed to do homework with friends记住： 数字与名词组成的复合形容词用‘ -’ 连接，名词不能加S， 这类形容词后面要接名词

要注意中英文用法上的区别。