They are ( ) students.A.thirteen years old B.thirteen-year old

C
thirteen-year-old,用短横连接变成一个单词,而且是形容词,通常作定语,修饰名词,再如：an eight-year-old boy 一个八岁的男孩
thirteen years old,三个单词,通常在be动词后作表语,如：He is thirteen years old.