区别就是两者词性是不一样具体的不同如下
year-old为形容词，中文意思是做定语;…岁的;
例句
He was charged with the kidnap of a 25 year-old woman.
他被指控绑架一名25岁的女子。
yearsold为名词，中文意思是
岁;年龄;
例句
Young children are beginning their formal education sometimes as early as four years old.
幼儿有时早在四岁时就开始接受正规教育
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.
