你好!用连字符的只有 He is a 20- year- old boy . 其它的都不对。不用连字符的:He is a 20 years old boy 或者He is a boy who is 20 years old
他是20岁男孩
He is a 20 years old boy
He is a boy at age of 20 years
He is a 20 years' age
1.He is a 20-year-old guy.
2.He is a twenty-year-old guy.
He is a twenty-year-old man.
He is a 20-year-old boy
He is a 20 years old boy