he is a 17 years old student

这句话 对吗？he is a 17-year-old student
答案说 he is a 17-year-old student.对，但为什么呢

he is a 6-year-old boy。可以说he is a 6 years old boy吗 He is a thirty-year-old man能否译成He is a man in his thirty ? He is __,and he has__sister.A.ten years old,an eight-year-old B.ten-year-old,eight year old He six-year old sister is as fit as a fiddle is he a peasant? he is a student? He is a doctor He was a good student last year Spring is the king of a year he is 18 years old.为什么year要加S