15-year-old 与15 years old 的区别有没有 15 years-old呢?

15-year-old 用作形容词，修饰名词；
He is a 15-year-old boy.
=He is 15 years old.

这两种表达方式都表示15岁。
没有15-years-old这种表达
第一个是单数形式 year不需要加s
第二种是复数形式 year需要加s

15-year-old 与15 years old 没有区别，都表示15岁
15 years-old是不对的

没有区别.
但也没有15 years-old这种用法.15-year-old被理解成一个合成词,所以year不用加S.

前一个做定语，She is a 15-year-old girl.
后一个做表语，She is 15 years old.
没有15 years-old这种表达.

15-year-old可以做定语，例如a 15-year-old boy
15 years old一般做表语多一点，例如He is 15 years old

15-year-old 可以做形容词,如 a 15-year-old boy; 15 years old 只能作表语,如he is 15 years old

前者是复合形容词，后面必须加名词，后者是可以放在句尾的比如You are a15-year-old boy=You are 15 years old 你后面说的情况没有噢