year old,year-old,years old,years-old的区别

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.