year old只能是 He is only one year old.即years old的单数形式. years old就是一岁以上的了,例如She is three years old. year old 和 years old通常都作表语. 首先要声明绝对没有years-old的说法,因为加上连接符以后year必须用单数. 数字-year-old表示几岁,与前两个最大的区别就是它在句中只能充当定语. 例如：a three-year-old boy一个三岁大的小男孩. 但是不能说The boy is three-year-old.也就是说它不能充当表语, 而要说 The boy is three years old.
